Use QSFPM::setFilterRegularExpression everywhere
Instead of setFilterFixedString() which doesn't modify the QRegularExpression part of the QSFPM, and use QRegularExpression::escape() on the pattern to make it plain text.
Match case-insensitively to match what's done in SortFilterModel::setFilterRegExp().
Note that this whole class is going to be moved out of here (see comment in header file).