editor: use getTrackFromX helper

Instead of sprinkling the knowledge of what apears where aound in
the code, let's use a helper instead. This way, we can more easily
change widths of single tracks.
editor: do not track width/height of TrackArea

We can ask the viewport about it instead.
editor: do not track windowTracks
editor: make invalidateTrack invalidate track-names

Previously, invalidateTrack only invalidated the track-data, but
this isn't correct. Some other invalidation seems to have kept
this problem under control, but this is about to change...
editor: per-pixel horizontal scrolling

This feels a bit nicer.
editor: separate between logical and physical coordinates