"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/daschuer/mixxx/commit/37c425845db4aeb7c747e3f8813537bfca8e584a
and it seams to work.