Add GObject type system integration

[sundermann]

We need to provide integration of the GObject type system into the .NET world and vice versa. We can generate the needed bindings from GObject-2.0.gir and then add the type system on top of it. Most code can be copied from gtk-sharp (basically anything but the interop stuff which can be autogenerated). We need integration for signals, inheritance, GType mapping and probably more stuff.

[sundermann]

This should probably go into a new library gobject-sharp. In current gtk-sharp the typesystem stuff is inside glib-sharp but I think this should be in separate libraries.