Add Ctrl-Alt-Del item back

[manics]

Proposed change

Ctrl-Alt-Del is removed in https://github.com/jupyterhub/jupyter-remote-desktop-proxy/pull/78 It's occasionally useful as an escape hatch so we should add it back, albeit in a menu instead of a top-level button

Alternative options

Do nothing

Who would use this feature?

Users who do something strange in their environment and need an escape hatch. User with custom/alternative desktops other than XFCE where Ctrl-Alt-Del may be more useful.

(Optional): Suggest a solution

Add it back somewhere

[consideRatio]

I saw the following related API for the RFB JS object: https://github.com/novnc/noVNC/blob/master/docs/API.md#rfbsendctrlaltdel