settings: Implement the contrast settings

[hfiguiere]

See flatpak/xdg-desktop-portal#1175

Note: I still have questions how this work on a non GNOME system.... so it might not be relevant.

Mostly a cut and paste of https://gitlab.gnome.org/GNOME/xdg-desktop-portal-gnome/-/merge_requests/143

[TingPing]

Note: I still have questions how this work on a non GNOME system.... so it might not be relevant.

This is the only setting GTK looks at to detect high contrast. Other desktops have to set the GSetting. KDE already sets various GSettings for theme, not sure if they set this one too but that would be changed there.

[dominichayesferen]

Note: I still have questions how this work on a non GNOME system.... so it might not be relevant.

This is the only setting GTK looks at to detect high contrast. Other desktops have to set the GSetting. KDE already sets various GSettings for theme, not sure if they set this one too but that would be changed there.

For the record, last I checked they don't... because they don't have High Contrast in that regard yet (until now, the most they have had is an ill-fated Breeze Dark colour scheme edit).