Debugger should be more visible

[nicoatsun]

Instead of having to set the VBOX_GUI_DBG_ENABLED env var to enable the VirtualBox VM debugger, one should only have to toggle a per-VM checkbox. And the debugger should be documented.

[Frank Mehnert]

I'm fear there will be no further documentation to the debugger than the built-in help as it is subject to frequent changes. And no, we will not make the debugger available using a check box because the debugger is some debugging / development aid and not useful / required for the normal VM. You still can set the environment variable for dedicated VMs.