port slideshow to Qt 6

[jriddell]

simple code copy so we have a Qt 5 and 6 version, although since the difference is only 1 line it might be better just to remove that one line.

[dalto8]

Maintaining parallel QML files seems like a nightmare.

[jriddell]

a nightmare but it's what is done with the qml modules

[adriaandegroot]

So, to be clear:

• makes a copy of all the QML things into a calamares-qt5 directory
• tries to pick one of the two directories at build time based on the WITH_QT6 flag

I would suggest renaming and copying, so we end up with a calamars-qt5/ and a calamares-qt6/ under the qml/ directory.

And yes, maintaining Qt5 and Qt6 QML is a nightmare because of the source-incompatibilities. It is a road already taken, though, with the modules. I'd be willing to entertain a Qt6-only Calamares version (3.4?) once KDE Frameworks 6 happens and the KDE Megarelease is done.

[jriddell]

@adriaandegroot yes that seems right. I renamed the directory as you suggested