The Group property for all the Radio* classes (RadioAction, RadioButton,
RadioMenuItem and RadioToolButton) was exposed as a GLib.SList,
with its elements untyped.
We now hide the various Group properties and have custom code to handle
them correctly.
There are no real code changes in this commit, just a lot of file
renaming and boilerplate additions.
A few .custom files are just removed, because the corresponding class in
GTK is gone, so they were not really used anymore.
Some files need to be re-indented, but that will be done in a separate
commit, so that git can track the renamed files correctly and not be
confused by all the changes.
