Sync the playheads across all editors


Remarks from #1195:

> With the piano roll if you press play in the dedicated window you can see a playhead/marker so you can see what is being played and where you are in the pattern. However, when you play the whole track from the timeline window and open a piano roll to edit something there is no playhead any more in the piano roll. Again, you have to resort to blind editing if you want to do some editing at the same time as hearing the other instruments.
>
> Basically I am requesting that you can see a playhead marker in any/all edit windows when playing the from timeline, assuming the clip is being played.

Start by reproducing playback from the timeline while a piano roll or other editor is open, then trace how the timeline and editor windows represent their playhead markers. Done means the marker remains visible and synchronized in every editor whose clip is currently playing during whole-track playback.

Written by the indexing model from the issue text.