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/
FYI this may result in a one-buffer error if this is called after one buffer is done processing but before the other is  (see my sync_fix PR).  Maybe that's good enough for this function?