Visualize the calculated print area in sliced view

[RichiH]

Is your feature request related to a problem? Please describe.
As a bit of a safe-guard against #13983 but also as a way to e.g. use partially broken print sheets (like mine is now...), it would be useful to see the print area which has been calculated and which the printer will probe before printing.

This might also help with more advanced use cases like printing on fabric, etc.

Describe the solution you'd like
I'd like to see a faint rectangle on what the sliced gcode will have as print area.