csd: Make link activation work reliably

The label code was just letting some button press events bubble
up, which caused misbehaviour with the window dragging code.
https://bugzilla.gnome.org/show_bug.cgi?id=695506

gtk/gtklabel.c

@@ -4597,6 +4597,8 @@ gtk_label_button_press (GtkWidget      *widget,
         {
           info->link_clicked = 1;
           gtk_widget_queue_draw (widget);
+          if (!info->selectable)
+            return TRUE;
         }
     }