Add Pioneer DDJ SX2 Mapping

[Krafting]

Initial implementation of the Pioneer DDJ-SX2 controller. Supports most features mentioned in the manual. (Some are not possible, for example the Flip controls)

Related documentation is at mixxxdj/manual#877

There is an area of the mapping I would love to get help with though, nothing blocking this PR, but if anyone would like to help, it's about the Slicer mode is just some beatjump rightnow, but it would be great to have a real Slicer

There is also the Slicer Loop mode that does the exact same as the current slicer mode (beatjump), I might use it for STEMs support later on when 2.6 comes around, or if anyone have any better ideas ! :)

By the way, the code lives here too : https://gitlab.com/Krafting/mixxx-pioneer-ddj-sx2

p.s. This is my first mapping contribution, please tell me if I'm doing anything wrong!

This is based on tildearrow's mapping ( https://github.com/tildearrow/Mixxx-Pioneer-DDJ-SX2 ), heavily modified by myself and eXWoLL1

[daschuer]

Thank you for this PR!
As a first-time contributor we need you to sign the Mixxx Contributor Agreement and comment here when you have done so. It gives us permission to distribute your contribution under the GPL v2 or later license and the Apple Mac App Store. It is also helpful for us to have contact information for contributors in case we may need it in the future.

[daschuer]

I will retarget to the 2.6 branch, because we will release 2.6 soon.

[Krafting]

Hi, thank you !

Quick question, do I need to resign the contributor agreement even if I already did with my contributions in the manual repo : mixxxdj/manual#875

Btw, I think the mapping is ready for review, it is in a state I'm comfortable sharing with the community!

p.s. can't wait for 2.6! \o/

[daschuer]

Ah I missed that. One signing is sufficient.