Worked around Metacity crashes in xcb plugin.

Setting the user time before mapping the window seems to prevent the
crasher from happening. We used to set the user time before mapping in
Qt 4.8 too, so it's probably the right thing to do.

Task-number: QTBUG-24462
Change-Id: Ia670b799bd1ed7a7e6399631d5242e57324918b3
Reviewed-by: Jan Arne Petersen <jpetersen@openismus.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -548,6 +548,8 @@ void QXcbWindow::show()
         updateNetWmStateBeforeMap();
     }
 
+    updateNetWmUserTime(connection()->time());
+
     Q_XCB_CALL(xcb_map_window(xcb_connection(), m_window));
     xcb_flush(xcb_connection());