This makes enough changes to make it compile with -Dkazv_KF_QT_MAJOR_VERSION=6.
Type: add
CCBUG: https://iron.lily-is.land/T85
Verify that unit tests pass and kazv still works with Qt5/KF5. Verify that it compiles with Qt6/KF6. Note that unit tests do not work yet with KF6.
Such a huge amount of work!