Fix the tech debt introduced by #24944

[cbjeukendrup]

PR #24944 was merged to keep everyone happy, except ourselves, because it introduced quite some tech debt.

• Remove staff index awareness from FluidSequencer
• Remove MIDI output awareness from FluidSynth
  (it was already there, but that PR made it even bigger)
  (see also Show playing notes on the piano keyboard #21874, which does some work in that direction)
• Clean up MidiEvent class
  The PR revealed that this class is mixing and confusing MIDI 1.0 and 2.0; it added some patchwork to fix that, but there seems to be room for a more structured solution.