* src/glade-catalog.c: read the name of the plugin to be used by each
	catalog and pass it to the constructor of glade-widget-class.  Also
	if a specific class has a specific plugin name to be used, use it instead
	of the generic of the whole catalog.
	* src/glade-gtk.c: remove the "static" from the functions that should
	be exported, and remove the "temp hack" that was used to load the
	address of the functions, as it's no more needed.
	* src/glade-property-class.c: remove glade_property_class_get_{get,set}_function
	and use g_module_symbol instead.
	* src/glade-widget-class.c: use g_module_symbol instead of the previous {get,set}
	functions.  Remove unused functions.  Rename glade_widget_class_new_from_name2
	to glade_widget_class_new as it's the only constructor.  Use the plugin
	name that glade-widget-class passed to the constructor and load it.
	* src/glade-widget-class.h: add a GModule* by GladeWidgetClass.
	* src/glade-widget.c: remove the #include <glade-gtk.h>
	* widgets/gtk-additional.xml: add the name of the plugin ("gtk") for all the
	widgets on gtk-additional.
	* widgets/gtk-base.xml: ditto
	* widgets/gtkwidget.xml: remove the "name" tag, as it's not need.
	* widgets/gtkwindow.xml: change glade_gtk_widget_{get,set}_tooltip by "ignore",
	as that's what these functions do.