* src/glade-property.[ch]:
  - made some members properties
  - added glade_property_set (property, ...) convenience function
  - Added set_tooltip & tooltip changed signal
  - Added insensitive property.

* src/glade-property-class.h: Clarified comment

* src/glade-property-class.c: Properties that are native to GtkWidget go in
  the "common" tab by default (unless overridden by the xml file).

* src/glade-editor.[ch]: Now we watch property sensitivity and tooltip status

* src/glade-gtk.c: The beginnings of stock buttons.