Hello guys, this patch replaces the old toplevel embeding hack by
extending the "parent" catalog class property.
This lets us make glade use a GtkEventBox as the runtime widget
instead of a GtkWindow.

The only downside is that we cant rely on the type of the object at
runtime, so i had to replace all the GTK_IS_WINDOW.

I will appreciate if you guys could test it and send me some feedback

greets

Juan Pablo