Knob arcs: add options for reversed paint direction & round caps

[ronso0]

Final additions to #2275 / #2686

• <ArcRoundCaps> allows to use a round pen cap for drawing the arc. For regularly sized knobs this will slightly increase visibility of the arc if the knob is just a bit away from the center. For large knobs it's a style tool.
• <ArcReversed> paints the arc from maxAngle to current position. I will use this for the microphone Ducking knob to communicate the ducking effect. Later on, we can use this for effect knobs like BitCrusher parameters.

Ducking: round cap
Mic Pregain: flat cap
See how the knob and the arc position diverge.

[ronso0]

Looking at Deere, I think it makes sense to also add a configurable 'center gap' for the background arcs. I guess this will complicate the arc code even more but I think it would be worth it to finalize the arcs for being configured automatically be the connected control later on (center gap, unipolar/bipolar, reversed).

[daschuer]

LGTM, Thank you.