only build QML with Qt6

[Be-ing]

This will allow us to use new features in Qt6 without worrying
about compatibility with Qt5.

Prerequisites:

• effect chain preset saving/loading, custom effect defaults, and parameter hiding/rearrangement #2618
• vcpkg manifest mode #4281 (CI builds with Qt6)
• [qtkeychain] switch to Qt6; add qtkeychain-qt5; update to 0.13.1 microsoft/vcpkg#20185 (CI builds with Qt6)

[Holzhaus]

Let's please not merge this unless we actually have an open PR that depends on a Qt6-only feature that we urgently need and can't be dealt with using a QT_VERSION_CHECK.

[Be-ing]

I do not care to check whether every new feature is compatible with Qt5. I will not waste my time continuing to develop the QML GUI with Qt5.

We need CI setup with Qt6 before we can merge this though.

[Holzhaus]

I do not care to check whether every new feature is compatible with Qt5. I will not waste my time continuing to develop the QML GUI with Qt5.

I do not care to reconfigure/rebuild mixxx all the time if I want to switch between QML/Qt6 development and regular mixxx development without any good reason. I will not waste my time continuing to develop the QML GUI with Qt6 only when it's completely unnecessary to drop Qt5 support at this point and just drastically worsen the developer experience. There are so many tasks that you can start working on right now without any issues, I just don't get why this is needed.

[Be-ing]

I do not care to reconfigure/rebuild mixxx all the time if I want to switch between QML/Qt6 development and regular mixxx development without any good reason.

Thanks to your work switching to CMake, this is not a problem. Just keep a separate build directory for Qt6. This is what I am doing now and it works well.

[Holzhaus]

It's still much less convenient than just adding the --qml argument to the command line. And it requires Qt6 to be installed, a second build dir, cd'ing around all the time, etc.
I'm not denying that it may be necessary to drop Qt5 support at some point, but I don' t understand why you needlessly want to raise the bar for contributing to QML now.

QML has its own versioning and is backwards compatible. So unless you absolutely need a certain feature only available in Qt6, just use QtQuick 2.12 (=Qt 5.12) to 2.15 (=Qt 5.15) and it will also work with Qt6.

If you have a particular PR that is too complicated to implement with Qt5 support we can consider merging this, but until then I really don't see a reason why we'd should make development less convenient.

[Be-ing]

it requires Qt6 to be installed

So? Qt6 is packaged in most distros now. If not, it's not hard to build. I built Qt 6.0 the day it was released.

cd'ing around all the time

This is not required. I have a build (Qt5) and a qt6 directory at the root of my mixxx repository. It is trivial to switch between them. It is simply a matter of running

cmake --build build

or

cmake --build qt6

[Be-ing]

If you have a particular PR that is too complicated to implement with Qt5 support

#4448