Add ability to draw track names and distance labels on the viewport.

Add ability to draw track names and distance labels on the viewport.

Track labels are drawn in the appropriate track colour, when set.

Options are given to draw the track names at:
 a centre position
 start and end positions
 centre, start and end

The default is to not draw them.

Also distance along tracks may be displayed with up to N labels at friendly distance amounts.

Originally I had worked(and reworked) over several differing commits,
 but too tricky to split into back into the resulting logical commits, so combined together.