Add filament unload status messages for frontend feedback

[medeirosT]

Right now UNLOAD_FILAMENT works, but from the UI side it's a bit of a black box.

The printer does the right thing, but the user has no idea if it's still cutting, retracting, or if it's already finished and waiting for them to pull the filament.

This adds status messages so GrumpyScreen (and other frontends) can show the same kind of feedback we already have during loading.

The goal is simple: stop users from having to second guess what the printer is doing.

Edit : If this is an approved approach, I'll go ahead and patch grumpy too

[suchmememanyskill]

I don't think agree with the changes here. Maybe a popup saying its done, but the printer should be in constant motion essentially. It isn't like the printer is sitting idle and you question what its waiting for. The console is also not really where many people look for whats happening, especially as its 2 menu's away from unload. SET_DISPLAY_TEXT might be better but the seuqence is so quick i don't know if it has any value

Maybe even better, recently the beeper got added. Maybe beep when its done?

[medeirosT]

Alternatively, on the discord, @suchmememanyskill suggested we use instead:

_SHOW_PROMPT TITLE="Unload completed" MESSAGE="Filament is safe to remove now"

I'll leave it to the devs, so I'll update the PR!