MIDI-172: MidiPiece.trim cleanup

[roszcz]

Summary of Changes:

• Refactored the MidiPiece.trim method:
  • Removed the slice_type and shift_time arguments to simplify the API.
  • The method now always trims notes using a standard time window and shifts note timings so the window starts at zero.
  • Removed support for 'index' and 'by_end' slicing modes.
  • Updated related logic in __getitem__ and removed time-shifting functionality from it.
• Cleaned up and updated associated tests:
  • Removed tests for deprecated slice_type, shift_time, and related edge cases.
  • Adjusted relevant assertions and thresholds in pianoroll viewer tests.
• Updated usage in pianoroll view to match the new trim method signature.
• Added a copy method to MidiPiece.

Overall, these changes simplify the codebase, clarify the API, and remove unused or confusing options in MIDI piece trimming.