Retain QML sources in deliveries
The app uses ahead-of-time compilation of QML sources to speed up start up times. However, currently, the way it is implemented, the original QML sources are not deployed along the app.
This causes the app to not start if e.g. the Qt version changes. This can happen e.g. in the Flatpak build, when the KDE runtime we use is updated.
To mitigate, we should include the QML sources in the app, by putting the appropriate resources files into the
QTQUICK_COMPILER_RETAINED_RESOURCES variable. See also https://doc.qt.io/qt-5/qtquick-deployment.html.