In singletop mode, all frames show the "Quit" option in the file menu
and will quit on Ctrl-Q. When launched from the main KiCad interface,
sub-programs show the "Close" option instead and will close with Ctrl-W.
In this mode, Ctrl-Q will instruct the main program to exit.
Fixes: lp:1779938
* https://bugs.launchpad.net/kicad/+bug/1779938