GdkDeviceManagerXI2: don't set user_time on ButtonRelease

The XI2 device manager was mistakenly setting the window user_time on
both ButtonPress and ButtonRelease, which meant that processes that
tried to launch another process based on the time of a ButtonPress
event would end up always focus-stealing-preventing the new app.

https://bugzilla.gnome.org/show_bug.cgi?id=647275

gdk/x11/gdkdevicemanager-xi2.c

@@ -1204,7 +1204,8 @@ gdk_x11_device_manager_xi2_translate_event (GdkEventTranslator *translator,
             break;
           }
 
-        set_user_time (event);
+        if (ev->evtype == XI_ButtonPress)
+	  set_user_time (event);
 
         break;
       }