[GLib] Force removal of timeout from proxy list.
[GLib] Force removal of timeout from proxy list.

This fixes a leak where if you removed a source via GLib.Source.Remove, you would end up with the SourceProxy in memory forever (i.e. TimeoutProxy), as it did not remove it from the dictionary caches in the implementations.

sillsdev/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp