Non-audio layers not accessible in zynmixer branch

[riban-bw]

Describe the bug ZynMixer branch presents layers as chains feeding channel strips in an audio mixer. Access to each channel is from the mixer but non-audio channels such as MIDI effects chains can not be easily accessed after they are created.

To Reproduce Steps to reproduce the behaviour:

1. Add MIDI effects layer
2. Try to access the MIDI effects layer

Expected behaviour Ability to access MIDI effects layer.

Setup:

• HARDWARE Raspberry Pi 3 Model B Plus Rev 1.3 Soundcard: HifiBerry DAC+ ADC PRO Display: ZynScreen 3.5 (v1) Wiring: MCP23017_ZynScreen GPIO Expander: MCP23017
• SYSTEM Raspbian GNU/Linux 10 (buster) Build Date: 2021-09-23 Memory: 20% (192M/923M) SD Card: 44% (12G/29G) Temperature: 55.8ºC
• MIDI Profile: default.sh Fine Tuning: 440 Hz Master Channel: off Single Active Channel: off ZS3 SubSnapShots: on
• SOFTWARE zyncoder: testing (7c09738) zynthian-ui: zynmixer (643aaa2) zynthian-sys: testing (b49f8e8) zynthian-data: testing (7ca0778) zynthian-webconf: testing (39b61bb)

Additional context It may be advantageous to present MIDI chains as empty channels, i.e. just the channel number in the legend strip to allow it to be accessed.

It is possible to access MIDI layers by pressing the LAYER button within the control screen to page through the layers.

[jofemodo]

This can be also useful for engines using multiple MIDI channels and a single audio output (setBfree, Aeolus). We could find a way of "grouping chains" (logic and visually) sharing a common audio output.

[riban-bw]

Fixed by presenting all layers in mixer view.