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
DataControlSource() needs to define a default constructor for the QMetaType requirements in Qt6.
All unit tests still pass except kiconutilstest.
Edited by Ahmad Samir