Make Ctrl+y copy the input line to the clipboard, or the currently
selected item in the menu.
The vi key to copy is used because Ctrl+c is already bound.
Alternative bindings are:
Ctrl+x to cut, but this also clears the line.
Ctrl+shift+c like in terminals, but this is harder to press, and this
commit works differently from terminals anyway since you don't manually
highlight characters, it just copies the whole line.
Alt+w, the emacs copy binding, but unless you use emacs this is weird
and unfamiliar.