'no filament loaded' setting for multi material.

I would like to be able to select 'none' or similar from the filament choice drop-down for each of my 5 XL tools. Should have just an empty box for the color, and maybe do intelligent things like flag an error if any empty extruder is set for use on a print.

This would be especially nice for when multiple instances of PS can have settings synced properly (#4898) and one can have one instance next to the printer updated when spools are loaded and the other remote in the design office.