Implement looping (and devamp)

[space928]

A lot of the UI already exists for looping support but it isn't implemented yet. Devamping (exiting a cue at the end of it's loop) should be done with a Stop cue in LoopEnd mode.

It might be interesting to add functionality to optionally play a portion of the sound cue before and after the loop section. But this can also be achieved by simply using another cue.