Fix filtering in the SortFilterProxy model used in KeysManager
This is the line-edit in the main window. This was broken while porting to QRegularExpression, commit 25d9df5c. setFilterFixedString doesn't set the QRegularExpression parts of QSortFilterProxyModel, this is fixed in Qt6, for details see: !14
Since this was using setFilterFixedString, use the pattern only so as to match plain strings and not regex to keep the same behaviour.