Add mid-song instruments change for MIDI importing

[Quent12b0]

Is your feature request related to a problem? Please describe. When importing a MIDI file with channels changing instruments mid-song, the program only use one instrument by channel for the whole song (except for percussion (10th) channel).

Describe the enhancement you'd like Make that the program can recognize and perform mid-song instruments change when importing a midi. I think it may be possible, as it already exists for the percussion channel (10th).