Alternative mapping for American Audio VMS2

I know there is a mapping for AA VMS2 by Groschi already in Mixxx, but that only maps controls found in both VMS4 and VMS2 controllers. The rotational knob that is exclusive to VMS2 is not mapped at all. In my mapping I use it for library and sidebar browsing. Detailed differences to the mapping are described in the XML file.

Since I also needed to adapt the script part for browsing, I use a new script file for VMS2. The included mapping just reuses the VMS4 scripts.

I also included the cleanup suggested by @mspike in pull request #161.

I tested this on Debian Linux with AA VMS2.