improve FX Select buttons mapping for Numark Mixtrack Pro FX

[Qwertie-]

Bug Description

With the default configuration/mapping, the FX select buttons have unexpected mappings. For example, currently in Mixxx, pressing "Echo" scrolls/selects the effects on deck one, while pressing phaser scrolls the effects on deck two. Pressing reverb adjusts the beat grid.

My expectation is that pressing an effect button like Echo would set the effect on both decks to echo. Which is it what it does in VirtualDJ and Serato. I can see how possibly mapping them to other functions could be useful, but I'd think that having the default mapping match what's printed on the buttons would make most sense.

I'm happy to try to put up a PR to change the mapping, just wanted to discuss first.

Version

2.4.1

OS

MacOS