[MIDI] add volume changing capability for each synthesizer

[umjammer]

• [ ] rococoa
• [ ] opl3
• [x] jsyn ... synthesizer has master volume property
• [x] garvill ... works sysex master volume! great implementation!

[umjammer]

https://github.com/umjammer/vavi-sound-sandbox/tree/rococoa-synth-volume