Improve ambiguous Mix Orientation buttons in Sampler Decks

[Daniel Schürmann]

The Symbols for the mixer orientation of the sampler decks are possible misleading.
This comes up with Bug #1161147.

I am not sure if the letters R/L are world wide understandable.
But I am sure that at least "M" is misleading even in English: Mute vs. Middle.

The thing is worst in Most skin where M is grayed out while R and L are highlighted in green.
This is more a hint for Mute than to Middle.

IMHO, we should change it to something like this:
↰ / ↱ / ↰↱ / ⌀

[tomtom (thomas-8)]

Or reducing to L/R:

L green = Left
R green = Right
both grayed = Middle

[jus (jus)]

"M" is not highlighted because it is the default state. This is like with all our other buttons and switches. The different states of the button are described in the tooltip. We may see a problem where, based on previous user feedback, currently none exists.

Nonetheless moving off hardcoded english based labels in the GUI is one of the goal for 1.12, so i'll add this task to the blueprint. Replacing the characters with some descriptive icons sound good to me.

[jus (jus)]

Fixed in Deere by displaying a crossfader icon with different positions of the fader visible (middle/left/right), depending on the current state of the control.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/6971