gladeui/glade-popup.c, plugins/gtk+/glade-gtk.c, src/glade-window.c:
	- Privatized members of GladeWidgetAction
	- Created glade_widget_action_class_new() and some accessors for better code in
	  the adaptor
	- GWActionClass is now on the slice allocator
	- Added glade_widget_action_set/get_visible()
	- Removed glade_widget_remove[_pack]_action()
	- Plugin makes actions invisible instead of removing them
	- Everything updated for new sealed api