Port away from Qt 5.14 deprecated QList methods

Summary:
Refactor FileIndexerConfig::excludeFilters() as it doesn't make sense
to me to convert to a QStringList to a QSet then to QStringList, if
there is a reason I couldn't fathom it.

Test Plan: The code compiles and unit tests pass

Reviewers: Baloo, meven, bruns, astippich, dfaure

Reviewed By: dfaure

Subscribers: alexeymin, kde-frameworks-devel

Tags: Frameworks, Baloo

Maniphest Tasks: T11553

Differential Revision: https://phabricator.kde.org/D26101