controllers: fix Numark DJ2GO2 Touch sliders and knobs

[fjordstrom]

Fix input for sliders and knobs to use single byte value, since all
are on single CC status message.

Fix direction of tempo sliders.

Resolves: lp1948596

[Swiftb0y]

Hey there, thank you for taking the time to address a bug within mixxx.

[Swiftb0y]

Great. So to confirm, irregardless of the actual slider direction in the mixxx UI, when you move the slider to the + region on the controller, the slider within also goes towards + (and vice versa)?

[Swiftb0y]

In order to merge your PR, we must kindly ask you to sign our contributor agreement. Thanks.
https://docs.google.com/a/mixxx.org/spreadsheet/viewform?formkey=dEpYN2NkVEFnWWQzbkFfM0ZYYUZ5X2c6MQ


[fjordstrom]

Great. So to confirm, irregardless of the actual slider direction in the mixxx UI, when you move the slider to the + region on the controller, the slider within also goes towards + (and vice versa)?

Yes, this is the intended effect. I re-checked the current branch head with default settings and it works.

[fjordstrom]

In order to merge your PR, we must kindly ask you to sign our contributor agreement. Thanks.
https://docs.google.com/a/mixxx.org/spreadsheet/viewform?formkey=dEpYN2NkVEFnWWQzbkFfM0ZYYUZ5X2c6MQ


Done, thank you

[Swiftb0y]

LGTM. Thank you very much.

[Swiftb0y]

Also quick question @fjordstrom:
In the bug report on launchpad, the reporter mentioned that their tempo faders don't center on 0%. This is somewhat common on individual units of cheap hardware. Does the incorrect center issue occur on your hardware as well?

[fjordstrom]

@Swiftb0y the incorrect center occurs on mine as well, as is caused by the same issue with parsing two midi messages as one single value. This pull request fixes this too (tempo sliders center properly at 0% )

[fjordstrom]

(for reference, I have tested the controller on release 2.3 with default settings and the controller scripts and mapping present in the release, as well as with the fix, and checked midi messages with midisnoop to verify consistency)

[Swiftb0y]

Ah, yes. Upon closer inspection of the XML I actually see the problem. My fault this slipped through the initial review. This makes sense. Thank you for clarifying and submitting this patch.