MIDI input: add key signature alterations mapping

Some times ago, I wanted to enter some music in G minor (or Bes major with sharps accidentals).
Using the current MIDI input, I have to continuously switch the accidentals combo box from sharps to flats.

To solve this, I added a key signature combo box to the Midi input panel, and modified the notes mapping according to a combination of the key signature and the value of the accidentals combo box.

For example, to enter Gminor music, you should choose "Bes Major (2 flats)" in the key signature combo box and sharps in the accidentals combo box. That way, when pressing the correct keyboard key, you will get flats for B and E accidentals, and sharps for the other accidentals (C, D, F, G).

Another side effect is that if you choose a Fes Major signature, pressing the B key will result in a "ces" note, which is what should be expected when using that kind of signature.