Cannot see which controller is selected for X-Y control

[riban-bw]

Describe the bug The value arc is coloured to indicate a controller is enabled for X-Y control. If a controller is at its minimum value then there is no arc and the indication is not visible.

To Reproduce Steps to reproduce the behavior:

1. Add an engine
2. Go to the engine's control screen
3. Set a control to its minimum value
4. Simultaneously bold press on the control and another control

Expected behaviour Both controls indicate they are enabled for XY control.

Actual behaviour The control set to minimum value has no indication it is selected.

Setup:

• HARDWARE Raspberry Pi 4 Model B Rev 1.4 Audio: ZynADAC Display: ZynScreen 3.5 (v1) Wiring: MCP23017_ZynScreen I2C: MCP23017@0x20, MCP23008@0x21, ADS1115@0x48, MCP4728@0x60
• SYSTEM Raspbian GNU/Linux 10 (buster) Build Date: 2020-09-05 Memory: 9% (185M/1872M) SD Card: 75% (21G/29G) Temperature: 53.5ºC Overclock: None USB Storage: 5% (9.7G/235G)
• MIDI Profile: default.sh Tuning: 440 Hz Receive Mode: Stage (Omni On) ZS3 Sub-SnapShots: Off MIDI to Output: Off Master Channel: off
• SOFTWARE zyncoder: testing (10bb3a0) zynthian-ui: testing (4b10d49) zynthian-sys: testing (af767d2) zynthian-data: testing (10e1f9f) zynthian-webconf: testing (90d04eb)

Additional context I did add a fix for this but it was disablef for some reason. Instead of, or as well as colouring the arc, the title and/or the value can be coloured. These are always displayed and hence the indication is always visible. An alternative would be to colour the background of the controller. This may be more obvious and perhaps more accessible.

[riban-bw]

Issue #1179 suggests a workflow that may resolve this. It uses menus to access the X & Y controller assignment seperately then a menu to access the X-Y view. This would mean we don't need to colour controllers because their config is done via menu. We may want to indicate that we are in the X-Y control selection mode but that would be the subject of #1179.