Make the Twister a 4 deck controller

[SamWhited]

This does not change any functionality by default except that the LED below each button automatically becomes an indicator letting you know the controller is centered.

It does, however, make the controller work with all 4 decks by rewriting it in ComponentsJS.
It lets you select independently which side of the controller is being used for what (as opposed to changing the entire controller between decks 1, 2 and 3, 4), so you could be making changes to decks 1 and 4 at the same time, for example.

This also paves the way for mapping the other layers (eg. to an effects controller) at a later date.

There is currently no manual page for this controller to update.

[JoergAtGithub]

Please do not replace semantic XML mapping by scripting if it is not technically mandatory. As semantic mapping is machine readable and easy to modify by users.

[SamWhited]

Please do not replace semantic XML mapping by scripting if it is not technically mandatory. As semantic mapping is machine readable and easy to modify by users.

It is not possible to make this a 4 deck controller (which is done as part of this PR) with only an XML mapping, nor will it be possible to add a full effects control system later, or improve the LED usage, which is what I'd like to do with this controller.

EDIT: I've updated the title to make it clearer what this does, as I'm assuming that is what got read that made it look like it was a change just to how the mappings worked.

Ok, if scripting is needed for functionality it's fine

[SamWhited]

Added a commit allowing the gain knob LED (for both the main gain and the decks) to be used as a peak indicator and a setting to enable/disable it and select the color of the indicator as well as a similar setting to use the tempo knob button as a beat indicator (this one is off by default to avoid flashing distractions, but could be used when beat syncing).

[SamWhited]

I've made a companion PR to document this controller here: mixxxdj/manual#747