widget: Add draw signal

For now, the draw signal is emitted by default from the expose event, so
widgets can chose to either implement the expose event or the draw
function.
This is for the transitional period from expose event to draw signal.

Note that for now subclasses can only implement the draw function when
none of their parents implemented the expose event.

gtk/gtkwidget.h

@@ -348,6 +348,8 @@ struct _GtkWidgetClass
 				gboolean          was_grabbed);
   void (* child_notify)        (GtkWidget	 *widget,
 				GParamSpec       *pspec);
+  gboolean (* draw)	       (GtkWidget	 *widget,
+                                cairo_t          *cr);
   
   /* Mnemonics */
   gboolean (* mnemonic_activate) (GtkWidget    *widget,
@@ -937,6 +939,9 @@ void       gtk_widget_set_has_tooltip       (GtkWidget   *widget,
 					     gboolean     has_tooltip);
 gboolean   gtk_widget_get_has_tooltip       (GtkWidget   *widget);
 
+gboolean   gtk_cairo_should_draw_window     (cairo_t     *cr,
+                                             GdkWindow   *window);
+
 GType           gtk_requisition_get_type (void) G_GNUC_CONST;
 GtkRequisition *gtk_requisition_new      (void) G_GNUC_MALLOC;
 GtkRequisition *gtk_requisition_copy     (const GtkRequisition *requisition);