[Users] Temporarily show otherwise hidden menu bar when Alt is pressed

[nvx]

Would it please be possible to make the hidden menu bar
(mainwin_menubar=0) temporarily appear when one presses the left Alt
key? To understand exactly what I mean please test this feature e.g. in
Firefox (View > Toolbars > uncheck Menu Bar, then press the left Alt
key).

Most of the time (at least for me) the menu bar only wastes
screen space, however, making it visible when necessary via F12 is a bit
cumbersome (not to mention the fact that sometimes I need to think hard
which key it actually is to display the menu bar -- several times I even
had to close CM and edit clawsrc to get it back ;) ).

Thank you.

nvx