app: add gimp_tool_widget_changed()

... which emits the "changed" signal, for use in subclasses to
notify the tool about widget changes not resulting from property
changes.

app/display/gimptoolwidget.c

@@ -281,11 +281,13 @@ gimp_tool_widget_properties_changed (GObject     *object,
                                      guint        n_pspecs,
                                      GParamSpec **pspecs)
 {
+  GimpToolWidget *widget = GIMP_TOOL_WIDGET (object);
+
   G_OBJECT_CLASS (parent_class)->dispatch_properties_changed (object,
                                                               n_pspecs,
                                                               pspecs);
 
-  g_signal_emit (object, widget_signals[CHANGED], 0);
+  gimp_tool_widget_changed (widget);
 }
 
 static void
@@ -362,6 +364,14 @@ gimp_tool_widget_get_focus (GimpToolWidget *widget)
   return widget->private->focus;
 }
 
+void
+gimp_tool_widget_changed (GimpToolWidget *widget)
+{
+  g_return_if_fail (GIMP_IS_TOOL_WIDGET (widget));
+
+  g_signal_emit (widget, widget_signals[CHANGED], 0);
+}
+
 void
 gimp_tool_widget_response (GimpToolWidget *widget,
                            gint            response_id)

app/display/gimptoolwidget.h

@@ -131,6 +131,8 @@ gboolean           gimp_tool_widget_get_focus         (GimpToolWidget  *widget);
 
 /*  for subclasses, to notify the handling tool
  */
+void               gimp_tool_widget_changed           (GimpToolWidget  *widget);
+
 void               gimp_tool_widget_response          (GimpToolWidget  *widget,
                                                        gint             response_id);