Edit Track marks in a playing Deck

There is a demand for editing track marks in a playing deck.

This is require for example to add a cue-out when using Auto DJ

This could be possible achieved by:
* https://bugs.launchpad.net/mixxx/+bug/408111 "Requesting "instant doubles" function"
* Controller Channel mapping, partly discussed here https://github.com/mixxxdj/mixxx/pull/6
* Prepare feature, possible part of http://www.mixxx.org/wiki/doku.php/gsoc2013ideas#advanced_track_management_organization_tools

Flow
* Start track in Deck A
* Instant Double to a free "preview deck"
* Map Controller to Preview deck
* Add Marks cues Loops etc.
* Copy Marks back to Deck A

If the exact position id not that important, a Cue editor on the Track over view will also helpful, but it might requires Mouse usage.

Competitor solution:
VirtualDJ 8's sandboxing: http://djworx.com/sandboxing-comes-to-virtual-dj-8/