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/
@default-kramer 
Sure, "useful and correct" is the optimum, that might be hard to reach. But It looks like we are on a good way towards it. 
"Adjust beatgrid to match another playing deck" is already real good. 

@ywwg isn't the playing deck picked in a four deck layout? 

> Should I change the code to make it a no-op if either deck is stopped?

IMHO not needed.