[feature] power down MMU2 motors when printing in Single mode

[khenderick]

When a print is started in Single mode, the whole print will use the same filament and there won't be any movements needed for any of the motors.

However, at this moment the motors are powered, consuming energy and getting hot.

It might be a good idea to power down the motors once the print started. Once the print is finished, the selector can re-home and start retracting the filament. The selector won't be moved either while it's loaded so can remember its position.

At the cost of a bit more noise at the end of a print the motors will consume less power, stay cool and it will probably also help for the power-loss/recovery feature.

To detect Single mode, the firmware could;

• Make use of a new g-code indicating that powering down the motors is OK
• Base on the "choose filament" question asked at the start of the print
• Scan the g-code for the next filament change and decide based on time or occurence