[Qgis-psc] QGIS grant report: Support building QGIS app on Qt 6

[Nyall Dawson]

Hi PSC,

I'd like to report the successful completion of this grant project.
(see https://github.com/qgis/QGIS-Enhancement-Proposals/issues/243 for
proposal details).

In a fortunate turn of events, I didn't encounter any of the
previously expected significant issues during this work and as a
result was able to proceed much further with the Qt 6 port then
originally proposed!

The proposal originally concerned only porting the "gui" and "app"
libraries to Qt 6, but I was able to complete also the "3d" and
"server" libraries too. We now are at a stage where the majority of
QGIS builds and runs without any significant issues on Qt 6. The
github CI setup has been updated to also run the c++ tests for gui,
app, server and 3d, and the majority of these have been fixed so that
they pass on the Qt 6 builds too.

In addition, some tests which were failing under Qt 6 revealed some
real QGIS bugs which have been fixed in the process of this work. (So
there's a direct benefit for the existing Qt 5 builds too!).

As previously reported, the next major step toward Qt 6 is enabling
python support for Qt 6 builds, and then running the full test suite
(with all the 1000s of python based tests) so that we can get a more
complete picture of exactly how stable and bug-free the Qt 6 port is.

Thanks again for the opportunity to undertake this work!

Nyall