vtl: rework session-needs-save logic

vtl: rework session-needs-save logic

* videotimeline now immediately marks the session dirty on
  every state-changing operation
* video-montor is polled every 2.5 seconds for menu-relevant state info
  (ontop, OSD,..) and every 30 seconds for full state (window pos, size,..)
* new interface to query the video-monitor for its state and wait for
  all replies - terminating the process and using ::wait() is no longer
  required
* Ardour> quit and Ardour > close-session does no longer close the video
  session to enforce video-monitor state sync but uses above new interface