debugger: add widget watchers by mouse click in the debugged application.

The option is hidden in the context menu.
Currently the application must be stopped.

src/plugins/debugger/idebuggerengine.h

@@ -93,6 +93,8 @@ public:
 
     virtual void reloadSourceFiles() = 0;
     virtual void reloadFullStack() = 0;
+
+    virtual void watchPoint(const QPoint &) {}
 };
 
 } // namespace Internal