Customisable Show Extra Keys screen

[damies13]

Is your feature request related to a problem? Please describe.
I am using a Macbook with a touchbar, as such many of the typical "PC" keys are missing, specifically the one annoying me is the print screen key that's used for screen shots in windows and linux.

Describe the solution you'd like
I would like to be able to customise the keys available in the Show Extra Keys screen, in my case i'd like to add the print screen key and some function keys but I imagine other users would have different keys that would be useful to them.

Describe alternatives you've considered
I haven't been able to find a work around yet, there is no way I am able to send this keystroke at the moment.

Additional context

Show Extra Keys screen:

[arekm]

Lack of ability to issue sysrq [1] requests for Linux kernel over novnc is a problem.

1. https://docs.kernel.org/admin-guide/sysrq.html