implement new feature "beats_translate_to_sync"

Described here: https://bugs.launchpad.net/mixxx/+bug/1180052

I can change all the skins if needed, but I thought 1.12 was going to ship with new skins, so I don't want to waste effort. Let me know.

Note that this feature can cause a (minor, in my opinion) sync bug to be more noticeable. I described the bug here http://sourceforge.net/p/mixxx/mailman/message/32711547/
I think "beatgrid" is a generic term.  Even if it's technically incorrect users shouldn't have to know the difference between beatgrid and beatmap.  The original tooltip is fine (yours also has a plural error: is/are)