Remove playHeadMarker static reference

[michael2402]

The MapView uses a playHeadMarker.

There is a special case handling for the MarkerLayer class in addLayers().

I would like to:
Step 1:

• Convert the PlayHeadMarker to be a temporary layer
• Remove MapView#playHeadMarker
• Remove PlayHeadMarker#playHead
• Move the whole addMouseListener code to hookUpMapView()

I'll be doing this between May 23 and May 27.

Step 2:
Remove the need for a mouse listener:

• Allow layers and the MapMode to intercept mouse events (MouseListener + MouseMotionListener) by implementing a new interface.

We currently have 12 uses of Main.mapView.addMouseListener. We can replace all of them by this change. This will also prevent us from having layers that do not unregister their events (like NoteLayer)
(no timeframe yet)