Qt-wide back functionality.

As forum.qt.io/post/779612 states, this is omnipresent in AOSP:

On certain windows like invent.kde.org/plasma/systemsettings/-/tree/bad848b014ecc5789c977d4db9b8f3154b5fe5cd, it would be nice to be able to go back sometimes to the previous view. However, to avoid reinventing the wheel, I’d like this for all Qt-based applications ideally.

This immediately seems impossible to place in the purview of the WM due to how each GUI toolkit manages such things differently, although Android manages to do so. Consequently, Qt seems the highest level that an implementation could be feasibly implemented. I don’t know whether anything as standardized as Android’s activity system exists in this case, but is it infeasible or undesirable to duplicate?

KDE already has a global menu bar, which is gaining support. It would be helpful to implement since its absence would be felt by anyone migrating from Android to Plasma Mobile (not that it is quite ready for prime time yet anyway).

Additionally, it has gained support in modern Windows toolkits, as the undermentioned demonstrate:

1. learn.microsoft.com/windows/apps/design/basics/navigation-history-and-backwards-navigation
2. stackoverflow.com/revisions/54743086/1
3. stackoverflow.com/revisions/76356293/1

I understand that consistency is more important for this functionality in the context of mobile platforms like AOSP, and know not what complexities a window manager would bring, so assign whatever broadness of scope appears feasible, if you consider the request to be.