Change the build system to enable building with Qt 6
This was built with:
-DQT_MAJOR_VERSION=6
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00
Move the include(KDEInstallDirs) call before the first find_package(Qt*, the former is what auto-detects the Qt version, and defaults to 5. This is needed to be able to build against Qt5 by default.
All unit tests still pass.
Edited by Ahmad Samir