Implement an "instant doubles" function

"Instant doubles" takes the track that is playing in one deck and loads it up on the other deck at the same place, it also copies across settings.

"Instant doubles" is available in several commercial dj softwares, it will be useful for a significant part of the Mixxx users (e.g. for beatjuggling or controllerism). Vinyl control users can switch turntables , mix using one turntable , use beatjuggling effects like echoing, doubling and phasing.

Objectives:
* Implement a "Instant doubles" function that copies the track, current pitch, keylock, position, cue points, loops, playback state etc. from the current deck to another one.
* Implement a "Auto instant doubles" option to the preferences which loads track automatically to both players on load.
This option should respect the "track load behavior" setting ( Load/don`t load tracks into a playing deck). We could remove the "Position display" option from then preferences to make some room, as that setting can already be changed directly in the GUI.
* Add a technical+user documentation for that feature.
* Add a keyboard shortcuts for loading tracks as instant doubles to another deck following the current keyboard mappings logic (CTRL+SHIFT+LEFT for player 1 / CTRL+SHIFT+RIGHT for player 2)
* Ensure it works for Vinyl control (VC) in relative and constant mode too.
* Ensure it supports n-decks so we could technically load doubles to other decks/samplers.

I am willing to pay a bounty for implementing that set of features into Mixxx.