Provide positionGeometryTip() as virtual method in AbstractClient

The implementation of positionGeometryTip is X specific, we need to
figure out whether that one makes sense for Wayland. Given that, let's
have it virtual to ease the transition of code which calls it.

abstract_client.cpp

@@ -1129,4 +1129,8 @@ bool AbstractClient::doStartMoveResize()
     return true;
 }
 
+void AbstractClient::positionGeometryTip()
+{
+}
+
 }

abstract_client.h

@@ -716,6 +716,7 @@ protected:
      * ensures that the internal mode is properly ended.
      **/
     virtual void leaveMoveResize();
+    virtual void positionGeometryTip();
 
     static bool haveResizeEffect() {
         return s_haveResizeEffect;

client.h

@@ -547,7 +547,7 @@ private:
     void leaveMoveResize() override;
     void handleMoveResize(int x, int y, int x_root, int y_root);
     void handleMoveResize(const QPoint &local, const QPoint &global);
-    void positionGeometryTip();
+    void positionGeometryTip() override;
     void grabButton(int mod);
     void ungrabButton(int mod);
     void resizeDecoration();