community/qt5-*: change header path to /usr/include/qt5

Previously the Qt5 headers were installed directly in /usr/include (e.g. /usr/include/QtCore/QString). With Qt6 having been released recently this needs to be moved to another directory so the Qt5 and Qt6 headers don't conflict.

This has been discussed with @PureTryOut (the maintainer) already. This needs to rebuild a bunch of packages (incl. qt5-qtwebengine), I've heard something about big rebuilds not being welcomed at the moment because of stable branching?