Fixed glib source double removal (#327)

Fixed glib source double removal then using Source.Remove (Timeout.Remove, Idle.Remove).
I don't know if fix is correct or safe, but it solves problem that you can test in TimerDemo section from samples.

Repro:
If on Windows you should enable console in samples: <OutputType>Exe</OutputType>
Go to TimerDemo section and press buttons:

1. Add timer
2. Remove timer by handler
3. GC - no error in console

1. Add timer
2. Remove timer
3. GC - error in console "GLib-CRITICAL **: 20:29:41.579: Source ID 123 was not found when attempting to remove it"

Source/Libs/GLibSharp/DestroyNotify.cs

@@ -47,6 +47,27 @@ namespace GLib {
 				return release_gchandle;
 			}
 		}
+
+		static void ReleaseSourceProxy (IntPtr data)
+		{
+			if (data == IntPtr.Zero)
+				return;
+			GCHandle gch = (GCHandle)data;
+			if (gch.Target is SourceProxy proxy) {
+				lock (proxy)
+					proxy.Dispose ();
 			}
+			gch.Free();
 		}
 
+		static DestroyNotify release_sourceproxy;
+
+		public static DestroyNotify SourceProxyNotifyHandler {
+			get {
+				if (release_sourceproxy == null)
+					release_sourceproxy = new DestroyNotify (ReleaseSourceProxy);
+				return release_sourceproxy;
+			}
+		}
+	}
+}

Source/Libs/GLibSharp/Idle.cs

@@ -48,15 +48,7 @@ namespace GLib {
 			{
 				try {
 					IdleHandler idle_handler = (IdleHandler) real_handler;
-
 					bool cont = idle_handler ();
-					if (!cont)
-					{
-						lock (this)
-						{
-							Dispose();
-						}
-					}
 					return cont;
 				} catch (Exception e) {
 					ExceptionManager.RaiseUnhandledException (e, false);
@@ -80,7 +72,7 @@ namespace GLib {
 			{
 				var gch = GCHandle.Alloc(p);
 				var userData = GCHandle.ToIntPtr(gch);
-				p.ID = g_idle_add_full (priority, (IdleHandlerInternal) p.proxy_handler, userData, DestroyHelper.NotifyHandler);
+				p.ID = g_idle_add_full (priority, (IdleHandlerInternal) p.proxy_handler, userData, DestroyHelper.SourceProxyNotifyHandler);
 			}
 
 			return p.ID;

Source/Libs/GLibSharp/Timeout.cs

@@ -46,15 +46,7 @@ namespace GLib {
 			{
 				try {
 					TimeoutHandler timeout_handler = (TimeoutHandler) real_handler;
-
 					bool cont = timeout_handler ();
-					if (!cont)
-					{
-						lock (this)
-						{
-							Dispose();
-						}
-					}
 					return cont;
 				} catch (Exception e) {
 					ExceptionManager.RaiseUnhandledException (e, false);
@@ -76,7 +68,7 @@ namespace GLib {
 			{
 				var gch = GCHandle.Alloc(p);
 				var userData = GCHandle.ToIntPtr(gch);
-				p.ID = g_timeout_add_full (priority, interval, (TimeoutHandlerInternal) p.proxy_handler, userData, DestroyHelper.NotifyHandler);
+				p.ID = g_timeout_add_full (priority, interval, (TimeoutHandlerInternal) p.proxy_handler, userData, DestroyHelper.SourceProxyNotifyHandler);
 			}
 
 			return p.ID;
@@ -103,7 +95,7 @@ namespace GLib {
 			{
 				var gch = GCHandle.Alloc(p);
 				var userData = GCHandle.ToIntPtr(gch);
-				p.ID = g_timeout_add_seconds_full (priority, interval, (TimeoutHandlerInternal) p.proxy_handler, userData, DestroyHelper.NotifyHandler);
+				p.ID = g_timeout_add_seconds_full (priority, interval, (TimeoutHandlerInternal) p.proxy_handler, userData, DestroyHelper.SourceProxyNotifyHandler);
 			}
 
 			return p.ID;

Source/Samples/Sections/Miscellaneous/TimerSection.cs

@@ -0,0 +1,84 @@
+using System;
+using System.Collections.Generic;
+using Gtk;
+
+namespace Samples
+{
+    [Section(ContentType = typeof(TimerDemo), Category = Category.Miscellaneous)]
+    class TimerSection : Box
+    {
+        public TimerSection() : base(Orientation.Horizontal, 3)
+        {
+            Valign = Align.Start;
+            TimerDemo.Create(this);
+        }
+    }
+
+    static class TimerDemo
+    {
+        public static void Create(Box box)
+        {
+            List<uint> timers = new List<uint>();
+            bool removeByHandler = false;
+
+            var btnAddTimer = new Button() { Label = "Add timer" };
+            btnAddTimer.Clicked += delegate
+            {
+                uint id = 0;
+                id = GLib.Timeout.Add(500, () =>
+                {
+                    ApplicationOutput.WriteLine("Timer tick " + id);
+
+                    if (removeByHandler)
+                    {
+                        removeByHandler = false;
+                        timers.Remove(id);
+                        ApplicationOutput.WriteLine("Remove timer from handler " + id);
+                        return false;
+                    }
+
+                    return true;
+                });
+
+                timers.Add(id);
+                ApplicationOutput.WriteLine("Add timer " + id);
+            };
+
+            var btnRemoveTimer = new Button() { Label = "Remove timer" };
+            btnRemoveTimer.Clicked += delegate
+            {
+                if (timers.Count == 0)
+                    return;
+
+                uint id = timers[0];
+                timers.RemoveAt(0);
+                GLib.Timeout.Remove(id);
+                ApplicationOutput.WriteLine("Remove timer " + id);
+            };
+
+            var btnRemoveTimerByHandler = new Button() { Label = "Remove timer by handler" };
+            btnRemoveTimerByHandler.Clicked += delegate
+            {
+                if (timers.Count == 0)
+                    return;
+
+                removeByHandler = true;
+                ApplicationOutput.WriteLine("Remove timer by handler");
+            };
+
+            var btnGc = new Button() { Label = "GC" };
+            btnGc.Clicked += delegate
+            {
+                ApplicationOutput.WriteLine("GC");
+                GC.Collect();
+                GC.WaitForPendingFinalizers();
+                GC.Collect();
+            };
+
+            box.PackStart(btnAddTimer, false, false, 0);
+            box.PackStart(btnRemoveTimer, false, false, 0);
+            box.PackStart(btnRemoveTimerByHandler, false, false, 0);
+            box.PackStart(btnGc, false, false, 0);
+        }
+    }
+}