Ability to disable ttymidi

[riban-bw]

Is your feature request related to a problem? Please describe.
The DIN-5 MIDI interface on zynthian is implemented with ttymidi service. Some custom builds do not expose the serial port and so this interface does not appear on the physical build but still appears in the list of MIDI interfaces. It would be advantageous to be able to disable ttymidi.

Describe the solution you'd like
The option in MIDI (Input/Output) Devices menu for DIN-5 MIDI to have a checkbox when navigated from admin menu, like the network and bluetooth devices. This checkbox would configure whether ttymidi was enabled.

Describe alternatives you've considered
Ignore it... but it keeps me awake at nights!

Additional context
We may need a configuration to record the required state that is used to start/stop the ttymidi service. The service should not be set to automatically start but instead be controlled by zynthian state manager. Need to decide whether this is saved with snapshots or as a global configuration. I prefer the latter.