convertToSingleTrackMidi unify interface for MID/MUS/XMI files

[Raffaello]

instead of using convertToSingleTrackMidi, name it getMidi and getMidi is getOriginalMidiFormat().

That in this case original format if it is format 0 is the same as getMidi.

but at least getmidi is used consistently among those "midi compatible" files.

create also a file_midi interface.

[Raffaello]

also

/Users/runner/work/sdl2-sonic-drivers/sdl2-sonic-drivers/sdl2-sonic-drivers/src/files/MIDFile.cpp:42:43: note: function declared non-throwing here
std::shared_ptraudio::MIDI MIDFile::convertToSingleTrackMIDI() const noexcept