add session save signal and hook-up videotimeline
fix prev commit SaveSession() signal was sent too late.
mark session dirty if video is moved
vtl: mark session dirty if video position-lock or timeline height changes
vtl: resume if "don't close/exit" is selected
vtl: disconnect from session-save signal

the videotimeline is a global unique instance that is allocated during
startup and deleted on exit. The Scopedconnection is not deleted
with the session.
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
update all i18n .po files
vtl: query video-monitor-state on explicit session save
Merge branch 'master' of git.ardour.org:ardour/ardour
fix LXVST support for plugins that make audioMaster callbacks from within effOpen (e.g. glitch)