We were already doing this for a key combination without modifiers,
but now we do the same for e.g. Control+Unknown. This matches the
behavior we have for QKeySequencePrivate::decodeString(), where
we return Qt::Key_Unknown if we can't resolve the key, even if
we have resolved some valid modifiers, e.g. "Meta+Trolls" as in
the tst_QKeySequence::parseString() test.
Change-Id: I238e29276e6ce356ae60c67585739587fa388f07
Reviewed-by: Liang Qi <liang.qi@qt.io>
ab99cf6077