Cleanup header usage in preparation for Qt 6

Espen Sandøy Hustadrequested to merge
ehustad/trojita:work/ehustad/cleanup_headers_for_qt6 into master

Some headers have been moved to a different module. Notable examples are for instance QStringRef. The current best practise is to not use the form <QtModule/class> or .

Co-authored-by: Matt Whitlock kde@mattwhitlock.name

This was split from MR !39 (merged).

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