Qt6 port plan : digiKam 8.0.0 and 7.x maintenance branch

[Gilles Caulier]

Hi all,

Following the huge commits list done this bugzilla entry:

https://bugs.kde.org/show_bug.cgi?id=416704

Whole digiKam code compiles under the Qt6/KF6 API from the "gsoc21-qt6-port"
branch. There is only few codes not yet compiled due to missing KF6
components:

- Marble
- libksane.

But it's just a question of time, and it will not block for the future...

Important remarks :

- Qt6::QMultimap does not inherit from QMap anymore. This means that we
need QMultimap and all iterators ported where it's needed. API has also
changed or moved in QMap in favor of QMultimap. Here of course regression
tests will be necessary to see if nothing is broken.

- There are plenty of warnings now in the console while compiling, and few
are really informative. This concerns Qt5 code (that we have ignored until
now). Typically, fixing Qt6 port warnings must be reported to Qt5.

- I haven't yet checked if this code still compiles with Qt5 in this branch
(it must). I'm sure that the minimal version of Qt5 is now 5.14.0 instead
of 5.9.x at least.

- One most important point is the code to merge in master : Where and when ?

I propose this plan :

1/ Move master code only compatible and mature with Qt5 in a
"qt5-maintenance" branch and use it later for next 7.6 and 7.7 releases. It
will be safe.

2/ Merge "gsoc21-qt6-port' to master. This code will be used for the main
development stream. digiKam version in master will be "8.0.0-git". Future
students will work from master as usual. They can work with Qt5 API in
respect with Qt6 compatibility. All students' work will not be backported
to "qt5-maintenance" branch.

3/ Backport main commits from master to "qt5-maintenance" branch when it's
necessary. Limit this branch to bug fixes now.

4/ Release digiKam 7.6.0 with "qt5-maintenance" branch in a few days or
weeks.

If there is no objection with this development workflow, I will process
changes in git during this weekend.

Best regards

Gilles
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