* cairo/Context.cs: add ctor (IntPtr, bool owned)
* cairo/cairo-api.xml: expose cairo_t as new ownable symbol type
* generator/HandleBase.cs: implement IOwnable
* generator/IOwnable.cs: new interface for wrapping ownable types
* generator/Makefile.am: add new files to build
* generator/OwnableGen.cs: SimpleBase subclass to expose IOwnable
* generator/Parameters.cs: use IOwnable.FromNative instead of HandleBase
* generator/Parser.cs: parse ownable symbol type
* generator/ReturnValue.cs: use IOwnable.FromNative instead of HandleBase
