o added GladeGetInternalChildrenFunc class and base implementation method
   o removed glade_widget_adaptor_get_internal_children() function
   o gwa_internal_children_update_from_node() reworked to work with simplyfied <internal-children> format
     <internal-children>
       <object/>
       <object>
         <object/>
       </object>
     </internal-children>

 * plugins/gtk+/glade-gtk.c:
   o fixed glade_gtk_container_get_children() now uses new get_internal_children
   GWA method and purify the list to remove posible duplicates

 * plugins/gtk+/gtk+.xml.in:
   o update <internal-children> tag to new format
   o make GtkTreeViewColumn:widget property parentless-widget

 * src/glade-window.c: set project in inspector after finishing loading
   (25% improvment in loading time)