Add robust Type 1 MIDI export

[TheWoomy27]

Summary

• Export a Standard MIDI File Type 1 conductor track with the tempo map and one track per LMMS instrument track.
• Preserve configured MIDI output channels, including channels imported from Standard MIDI Files.
• Serialize MIDI tracks dynamically so long tempo automation curves are not truncated by the former fixed buffer.

Testing

• cmake --build build --target midiexport midiimport MidiFileTest --parallel 4
• build/tests/MidiFileTest
• tests/scripted/verify

Standard MIDI Files represent tempo changes as discrete events. Linear and cubic LMMS tempo automation is sampled at sixteenth-note resolution.