Qt6 port of kconfig qconf

[Boris Kolpackov]

Is there interest in the Qt6 port of qconf (which is currently Qt5)?

I took an exploratory stab at it and I've managed to make it compile
and run with relatively few changes, though there are quite a few
warnings about using deprecated stuff.

I can see three possible approaches here:

1. Drop Qt5 support and port qconf to be exclusively Qt6. This should
   allow cleaning up all the deprecated stuff.

2. Support Qt5 and Qt6 in the same qconf.cc. With this approach it's
   unlikely we will be able to cleanup the deprecated stuff without
   turning it into an #if soup.

3. Leave qconf.cc to be Qt5-only and make the changes in the qconf6.cc
   copy. This will allow us to clean up all the deprecated stuff but
   will require applying further changes to two files until we drop
   support for Qt5.

Thoughts?