Clef changes along the score

New option for MIDI import - automatic clef changes along the score.

The algorithm:

for every chord
- if bass clef is current and chord average pitch (CAP) > 62 => change clef to treble
- if treble clef is current and CAP < 57 => change clef to bass
- if treble clef is current and CAP >= 57 && CAP < 60: if more than 3 subsequent chords satisfy this condition => change clef to bass
- if bass clef is current and CAP <= 62 && CAP >= 60: if more than 3 subsequent chords satisfy this condition => change clef to treble

Clef changes option is unchecked in the MIDI import panel by default.