Ensure MIDI devices show in connected devices dropdown

[neilenns]

Fixes #1944

There were two issues:

• The event handler for MIDI device lookup finishing was getting set after MIDI device manager was starting. This was a race condition, and on my computer the MIDI lookup was finishing before the event handler was added so it was never called.
• The code to actually add the menu items was adding things to the joystick menu instead of the MIDI menu.

The race condition could also happen for Joysticks so I did a minor refactor to create specific start methods to kick off the Joystick and MIDI managers, and only call them after the event handlers are added.

[github-actions]

Build for this pull request:
MobiFlightConnector.zip