Convert QML for Qt 6/KF6 compatibility when building for Qt 6

This is an alternative to the commonly used CMake preprocessing approach. It's slightly more flexible and could potentially be extended for supporting inline alternative code blocks for more complex changes.

The main advantage over the CMake approach is that the QML in the source dir remains valid and can be consumed by qmlimportscanner/androiddeployqt, and thus not breaking APKs for Qt 5.

The price we pay for that are in-place changes in the source dir, which is rather ugly.

The actual changes included so far are basically what is in !171 (merged) and !173 (closed), but on their own are not yet sufficient to start up on Qt 6. In particular the Kirigami page actions and the barcode scanner still need work.

Not sure if this is really the better approach here though, might be that a branch is easier after all.

cc @mlaurent