Introduce ClientConnection::aboutToBeDestroyed

This signal is useful if the compositor wants to perform some cleanup
before the disconnected signal is emitted or while the connection object
still has valid wl_client native handle.

src/wayland/clientconnection.cpp

@@ -68,6 +68,7 @@ void ClientConnectionPrivate::destroyListenerCallback(wl_listener *listener, voi
     Q_ASSERT(it != s_allClients.constEnd());
     auto p = (*it);
     auto q = p->q;
+    emit q->aboutToBeDestroyed();
     p->client = nullptr;
     wl_list_remove(&p->listener.link);
     emit q->disconnected(q);

src/wayland/clientconnection.h

@@ -123,6 +123,10 @@ public:
     void destroy();
 
 Q_SIGNALS:
+    /**
+     * This signal is emitted when the client is about to be destroyed.
+     */
+    void aboutToBeDestroyed();
     /**
      * Signal emitted when the ClientConnection got disconnected from the server.
      */