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/
It's hard to come up with a useful description that is also technically accurate. In which case, we should favor "useful to the user" over "technically accurate". Honestly, I didn't like it when I wrote it, and I don't think any of the suggestions are much better.

The use case is "I've nudged this deck into sync with another deck. I want to shift the beatgrid (of either deck) to reflect the nudging I've just done." I think an ideal tooltip would convey this use case. Perhaps "Adjust beatgrid to match another playing deck" is better? (And it's generic enough that it's not technically inaccurate regarding the single beat/multiple beats.)

And technically, this function works even if neither deck is playing. I don't see any usefulness to using this on stopped decks, but I didn't see a reason to disable it either. Should I change the code to make it a no-op if either deck is stopped?