Improved Tracks (simpler numbering, faster inserting, prevent deletion of locked tracks)

[jonoomph]

This is a pretty big re-write of the Track / Layers logic, with respects to how we number them, and how we insert, add, and remove them. The tracks numbering is now spread out:
Track 5 (number: 5,000,000)
Track 4 (number: 4,000,000)
Track 3 (number: 3,000,000)
Track 2 (number: 2,000,000)
Track 1 (number: 1,000,000)

Inserting a new track now simply picks a number halfway between the gap... for example:
Track 5 (number: 5,000,000)
Track 4 (number: 4,000,000)
Inserted Track (number 3,500,000)
Track 3 (number: 3,000,000)
Track 2 (number: 2,000,000)
Track 1 (number: 1,000,000)

With this logic, no other data is required to change, and it all magically works. However, if a user inserts around 30 tracks at the exact same spot, the 1/2'ing will eventually run out of a gap, and will then renumber things that conflict (similar to the current logic).

Also, disabled tracks are now blocked from the "Delete" context menu.

Otherwise, the behavior of Tracks is identical to before, just faster, simpler, and more stable.

[jonoomph]

I've updated this PR to also fix some issues with "Add to Timeline" and "Properties" views not display the correct track name (or customized track name). We should now be correctly displaying track names everywhere they are found.