Mixxx controls: rename categories, sort by context

[ronso0]

I frequently had trouble finding the correct controls category. For me, the issue is less severe because mostly I roughly know what I'm looking for and simply use the browser search. Furthermore the current group string formatting decreases readability IMO.
For new users it's appearantly bit more diificult, see lp:1941027 mixxx control for Big Library

Sparked by the conversation I started mixxxdj/mixxx#4355 but introduces at least one regression.

We could make the controls more discoverable if we didn't use the actual control groups ([Library], [Master]) but sort the controls by context. [Master],maximize_library would the go to "GUI controls".
For controls that are created only on request by skins we could mention that they'd only work with official skins.

vs.

Categories could be

• Sound configuration & Mixer
• GUI configuration
• Deck, ...