Make more use of XDG Settings portal

Not really a bug, more of a suggestion/conversation starter.

Currently for desktop integration on Linux we have QGenericUnixTheme with subclasses for Gnome/GTK and KDE. These mostly rely on reading desktop/GTK-specific things. The whole thing is a bit messy and complex, and not very generic. This is largely because there's nothing better available.

A better way to do this can be the Settings portal from XDG desktop portals. It offers a DBus API to read settings in a desktop-agnostic way. While it mostly exposes desktop-specific settings from Gnome and KDE it also standardizes a numer of generic settings. One of theme is the dark/light color preference, which we already use.

There's currently three such standard settings: color-scheme, accent-color, and constrast. However there are proposals for more such keys, see

https://github.com/flatpak/xdg-desktop-portal/pull/1590

https://github.com/flatpak/xdg-desktop-portal/pull/1539

https://github.com/flatpak/xdg-desktop-portal/pull/996

https://github.com/flatpak/xdg-desktop-portal/pull/1530

https://github.com/flatpak/xdg-desktop-portal/issues/1591

Some of the proposed ones would be useful for Qt to replace/augment existing stuff or expose them as new API.

Furthermore we as Qt developers also can and should take an active role in the standardization of these keys upstream and request what would be beneficial to Qt.