Mixer: add a "Clear All" button to reset all Solo and Mute buttons to default (disabled)

A suggestion based on a post on the Development and Technology Preview forum:
https://musescore.org/en/node/334126

1. In the Mixer, select several channels as Solo - for example all the Woodwind instruments in a symphony orchestra.
2. This is useful for listening only to a single section of instruments.
3. Now try to reset the Solo settings to disabled: it it necessary to click each Solo button separately to do this.

Suggestion: introduce a new "Clear All" button on the left of the row of S and M buttons
Advantage: speed up the workflow