Make QRegularExpression filtering actually case-insensitive

Summary:
I wrongly thought QSFPM::setFilterCaseSensitivity() would work when a
QRegularExpression was used, but that method only pertains to QRegExp.
Instead to get case-insensitive filtering set QRegularExpression::CaseInsensitiveOption
on the QRegularExpression object used with setFilterRegularExpression().

Also, thinking about this some more, no need to use QRegularExpression::escape()
with the QuickSelectDialog since only (\w| ) are accepted by the dialog,
everything else is gobbled up by the eventFilter().

Test Plan:
make && ctest, all unit tests pass except for:
tests-units-akonadi-akonadistoragetest
tests-units-akonadi-akonadicachingstorageintegrationtest
tests-units-migrator-zanshin021migrationtest
tests-units-migrator-zanshincontextitemsmigrationtest

which fail locally but not on the CI.

Reviewers: ervin

Reviewed By: ervin

Differential Revision: https://phabricator.kde.org/D26795