MIDI Player improvements - Start and Stop pins/markers

[BigHonkinDaddy]

When writing arrangements and when rehearsing sections of a score, I frequently need to repeat, over and over, a section of the score. It would be very useful and a time saver to be able to pin a "start" and "stop" point in the score that makes it easy to have the MIDI player re-start from the start pin and play until the stop pin. And maybe an easy way to position the MIDI player back to the start pin ready for another play/loop.

[cameronwhite]

Somewhat related to #117

[Ebdim9th]

and PT already has part of the training options already where you can adjust the overall percentage of the speed of playback regardless of internal tempo changes.....

[cameronwhite]

Yep, I think the main stuff left is having a start / end marker to loop between, and maybe also options to adjust the speed gradually on each loop