* src/glade-app.[ch]: Added show_properties method to app_class, this way
	  the implementing application can provide an implementation for show_properties.

	* src/glade-project-window.c:
	    - Implemented show_properties method
	    - Linked the update_ui_cb directly to app_class->update_ui_signal
	    - Removed ugly reference to editor->project_window

	* src/glade-popup.c: Call show_properties upon "properties" popup click.

	* src/glade-editor.h: Removed project_window from instance struct (useless and a PITA
	  to set).