[qtwayland] Add an extension for sending keys to specific windows.

The surface parameter of postQtKeyEvent is used to derive the target application, the actual window which receives the event is determined by focus on the client side.

Add new event which includes a parameter specifying the target window.

I'm not entirely sure whether this should be a new event in the qtkey extension, or another extension altogether? A new extension would make the event more independent but maybe that's just complexity for complexities sake.