Re-add the MIDI channal assign spinner to the mixer to avoid pizzicato and tremolo overlaps

In Musescore 3, you could set what channel you wanted an instrument to be assigned to and then it got removed in Musescore 4. This feature is vital if you exporting MIDI with a lot of bowed string samples because sometimes the MIDI files will prioritise pizzicato and tremolo channels even when you score does not have any of these. This often leads to different instruments being assigned to the same channel even though there are enough MIDI channels to take all of the instruments.

(The MIDI inclosed has the wrong programs set due to channel overlap from the score below, I believe this is due to the pizzicato and tremolo channels. I even tried the export musicXML file to manually assign the channels, import to Musescore and it still didn't assign them properly)