Mixer improvements

Here are my suggestions:

• Easier "batch" mute/solo-ing: Either by being able to manually create channel-groups (e.g. "Winds", "Strings", ...) and/or being able to easily select multiple channels and mute/solo all at once.

• Group instruments/channels per staff. Let's say I want to mute/sole a single staff (e.g. Violins I) I have to go through ALL articulations, and if there are instrument changes on the staff (e.g. Flute <> Piccolo) even MORE channels appear in the mixer, one more for each instrument change. It would be great to make this a bit more manageable and user-friendly.

• A minor flaw: Let's say I mute a channel and then press space to start the playback. Then the space bar unchecks the mute-checkbox. I have to focus the main window first. I know this is kinda default-behavior, but this can get pretty annoying very quickly. Maybe there is a way to work around that. Maybe don't use checkboxes at all, but stateful buttons instead.

I know there are other good topics including great improvement suggestions for the mixer. (Maybe they can be merged into one.) Here are the links:

https://musescore.org/en/node/262126
https://musescore.org/en/node/260971
https://musescore.org/en/node/256051
https://musescore.org/en/node/252986