sgreg/4chord-midi

UI redesign: add information about the four chords used in the selected key and highlight which one is played; show metre and make selectable. Functionality changes: Add 1/8th note playback; adjust playback modes and add a few more; add long press to menu buttons; clean up unnecessary USB MIDI descriptors. Add bootloader to easily update future firmware.

Extend configuration menu: enable/disable selectable keys and mode; add presets for user-definable fixed key/tempo/mode/metre selection; toggle between normal and preset mode; add playback "toggle on button press" playback modes as alternative to "play while button pressed" mode.

UI addition: add configuration menu structure and generic font for it. Add user-defined configuration, stored on EEPROM, and handling for: default key/tempo/mode/metre; name for notes between A and C (A#/Bb/B, B/H), backlight brightness; UART baud rate; humanizing options for playback.