Add 'Empty' to system filment preset dropdown

[KryptonUK]

A feature request to add an 'empty' option to the system list of filaments for use with the XL.

I set the filament presets for each of the XL heads, but if I've only got one or two filaments loaded it would be useful to mark which heads are empty so if I start a print remotely I can see what I'd left in each position.

For example, if I have three colours of Prusament PETG loaded into heads 1, 2 and 4 but then remove the filament in head 2, it would be useful to mark that as 'empty' in slicr so help remind me what is left in the printer.

Either adding a system preset as 'empty' at the top of the dropdown list, or clicking the text 'System Presets' at the top of the current list to clear the filament in that drop down would be very helpful.