AFP: Add loopback point

So this introduces an additional knob (and draggable point in wavegraph) which controls the loopback point.

This is very useful and necessary for sample-based work - without it, you can't have a separate "starting period" and "loop period" in the sample. So now, the sample loops between the loopback point and end point. Works fine with stutter mode too. 

Keeps backwards compatibility: old presets get loaded with the loopback point set to same point as the start point.

The knob is in a stupid position now but that's temporary until Tres finishes the needed graphics changes.