Make start/end/loop knobs responsive to each other

Per discussion in #1134.

The commit also contains some fast forwards ~~which I believe should only appear in the commit, not the pull (right?)~~.

@diizy if you can doubly check that the fast forwards don't cause issues that would be great.   I struggle with fast forwarding my fork so I edited from upstream and copied the changes to my fork, now the fast forwards are showing in my fork.  :monkey_face: 

-Tres