all commands reffer to that structure instead of trying to pull a project pointer from
	  a widget.

	* gladeui/glade-project.c: Unset project pointer on widgets when they leave the project,
	  manage an "in_project" flag on widgets to differenciate the moment when they widgets are
	  loading and have the project pointer but are still not added to the project yet. Also speed up
	  glade_project_has_object() by not traversing the list but simply checking the widget's project
	  and in_project flag.

	* gladeui/glade-widget.[ch]: Added the "in_project" flag to GladeWidget, fixed glade_widget_rebuild
	  to save the previous project pointer.