"no-op" refactor of slider logic into an event handler

Move all of the logic for slider mouse event handling into a separate class, like with knobeventhandler.  This will make it easier to test sliders and add features later.

I had to clean up a lot, so it's not just a cut-and-paste.

TESTED=horizontal and vertical sliders.  Did not test midi interaction to confirm I didn't break anything there
Removed in https://github.com/mixxxdj/mixxx/commit/d2fb971b2b81c3e1f5ef7f7418277b392f0edc7c