Handle SelectionWindowDestroy in QXcbClipboard

This change is related to 6a7ee92b39
which added handling for SelectionClientClose. Further testing showed
that with e.g. Qt 4 applications the SelectionClientClose is not
emitted, but the selection window seems to be destroyed before the
client is destroyed.

Fur a destroyed selection window the same applies: the clipboard
content is no longer valid and we should emit the changed signal.

Change-Id: Id3778a28b9f5601bf2c6e0106981316e0efa6e7c
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>

src/plugins/platforms/xcb/qxcbclipboard.cpp

@@ -742,7 +742,8 @@ void QXcbClipboard::handleXFixesSelectionRequest(xcb_xfixes_selection_notify_eve
             m_xClipboard[mode]->reset();
         }
         emitChanged(mode);
-    } else if (event->subtype == XCB_XFIXES_SELECTION_EVENT_SELECTION_CLIENT_CLOSE)
+    } else if (event->subtype == XCB_XFIXES_SELECTION_EVENT_SELECTION_CLIENT_CLOSE ||
+               event->subtype == XCB_XFIXES_SELECTION_EVENT_SELECTION_WINDOW_DESTROY)
         emitChanged(mode);
 }