[KeySequenceItem] Display ampersands in shortcuts
We have to replace the single ampersand that we get back from
QKeySequence::toString()
to be two ampersands, because the QQC2 Button
that will display this string interprets a single ampersand as a
mnemonic and hides it, which is not correct for our use of it here.
Shortcut buttons can now display ampersands:
BUG: 437718 FIXED-IN: 5.83
cc @davidre