* gladeui/glade-widget-adaptor.c: added GtkWidget::event signal clash override
    for every registered class to marshal events to the corresponding GladeDesignLayout

  * gladeui/glade-design-layout.[ch]:
    o added new function derived from glade_design_layout_widget_event(),
      glade_design_layout_do_event() to marshall events from the override class handler
    o removed placeholder from glade_design_layout_find_inside_container()

  * gladeui/glade-widget.[ch]: moved IS_GLADE_WIDGET_EVENT macro to header.