Qt6: Import Qt6 ports form kde@'s repository

This change brings in the new Qt6 stack, which might still be a bit
in flux in the ports tree for a while.

Obtained from https://github.com/freebsd/freebsd-ports-kde/tree/qt6-6.3.1
with commits from lbartoletti and myself.

Note:
ww/qt6-webengine will land once kai wins another round of patching boundled
chromiums sources.

Exp-run by: antoine
PR: 265843
Differential Revision: https://reviews.freebsd.org/D36179