Research reactive/adaptive layout for Qt Quick

We see a lot of value in letting applications adapt to different screen layouts, especially changing screen width. On the web there are various css frameworks such as bootstrap, that can be inspiration. This is purely a research task, nothing has to come out of it, but ideally we enhance layouts or create new ones to allow more flexibility in these scenarios - making it convenient for applications to adapt from mobile to hi res screen for example.

It would also be nice that a responsive layout (that becomes taller when its width is shrunk) can be dropped into a Flickable so that the tall content can be scrolled (or if the layout aggregates the Flickable in order the achieve the same functionality)

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Some inspirational links:
 * [Material 3 responsive UI|https://m1.material.io/layout/responsive-ui.html]
 * [CSS responsive grid system|http://www.responsivegridsystem.com/]
 * [Android Adaptive Scaffold|https://pub.dev/packages/flutter_adaptive_scaffold]
 * [https://blog.felgo.com/cross-platform-app-development/the-ultimate-guide-to-responsive-design-for-multiple-screen-sizes]

Qt blog: https://www.qt.io/blog/responsive-layouts-in-qt 

This is a follow up to https://bugreports.qt.io/browse/QTPM-1477 which was giving us some trouble.