Add loop modifiers: back, halve, double, forward

Adding simple modifiers to the current loop for moving it forward/backward "a step" and doubling/halving its duration Like if you're looping bars 5 to 8, you can move it to 1-4 or 9-12, and change to bars 5-6 or 5-12 by a single click. This is similar to UI of Amazing Slowdowner. I recently stopped using that app because it no longer supports Spotify streaming (which is not supported here either, but I like this YT interface a lot 😄