fix bug #19075 and #19080. make sure to handle the 2 + 3 button press events in nsWidget::HandleEvent().

widget/src/gtk/nsWidget.cpp

@@ -1250,6 +1250,8 @@ nsWidget::HandleEvent(GdkEvent *event)
       OnMotionNotifySignal (&event->motion);
       break;
     case GDK_BUTTON_PRESS:
+    case GDK_2BUTTON_PRESS:
+    case GDK_3BUTTON_PRESS:
       OnButtonPressSignal (&event->button);
       break;
     case GDK_BUTTON_RELEASE: