Enclose Qt5 and Qt6 blocks in their own scopes

To prevent the various KDE_(FULL_)INSTALL_* variables leaking out
put Qt5 and Qt6 specifics into a function and with it in their own
blocks.
Following on the outside we can't refer to them anymore so only install
the cmake config when the Qt6 is build which is the 'proper build'
compared to style support only with Qt5.
BUG:473292