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 should have been singular, because not "its beats" are aligned, it is only one beat aligned, the same as at left click.
The only difference it stat not the current position is taken into account, but the closes beat at an other playing deck. 
 An other try:
"While playing, adjust beatgrid so the closest beat is aligned with the closes beat on another playing deck."