QSortFilterProxyModel::setFilterFixedString() internally uses QRegExp
with QRegExp::FixedString syntax option; setFilterFixedString() was
ported by using setFilterRegularExpression(QRegularExpression::escape(string)),
this isn't perfect since QRegularExpression::escape() follows Perl quotemeta
algorithm, basically it escapes all characters in the pattern except for [A-Z],
[a-z], [0-9] and _; so for example a space ' ' is escaped too. See QRegularExpression
docs for the details.
Replace QSortFilterProxyModel::filterRegExp() with filterRegularExpression(),
the same for the relevant setters. Raise min. required Qt version to 5.12,
as that is where filterRegularExpression() was introduced.