glib: move some GLib.Object private methods to ClassInitializer class
glib: move some GLib.Object private methods to ClassInitializer class

Refactoring: moving these methods from GLib.Object to ClassInitializer
brings some benefits:
* We can mark OverrideHandlers as private instead of internal.
* We reduce the number of parameters to zero by making them use fields.
* We can make the god GLib.Object class a bit smaller.
* We can make the ClassInitializer.Idx counter private instead of internal.

akrisiun/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,orion75/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,Gankov/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,akrisiun/gtk-sharp,akrisiun/gtk-sharp,sillsdev/gtk-sharp,antoniusriha/gtk-sharp,antoniusriha/gtk-sharp