Track all MIDI messages for PerformedPart objects

[CarlosCancino-Chacon]

This pull request allows PerformedPart objects to track all MIDI message types. In particular it allows for tracking the following information

• key signature
• time signature
• Meta MIDI messages

While this information is not necessarily reliable in performed MIDI files, it might be sometimes useful, in particular for generated music/performances.

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (8ee8ba9) 85.19% compared to head (cfe0917) 85.18%.
Report is 2 commits behind head on develop.

Files	Patch %	Lines
partitura/io/exportmidi.py	28.57%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #343      +/-   ##
===========================================
- Coverage    85.19%   85.18%   -0.02%     
===========================================
  Files           81       81              
  Lines        13925    13949      +24     
===========================================
+ Hits         11863    11882      +19     
- Misses        2062     2067       +5     

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[manoskary]

This PR adds Meta Messages about time tick, Key signatures and Time Signature in Midi performances for import and export. Its overall and nice and simple addition for completeness. It adds some typing and minor documentation