Implement reduced motion setting

[leolost2605]

See https://gitlab.gnome.org/GNOME/xdg-desktop-portal-gnome/-/merge_requests/236 and flatpak/xdg-desktop-portal#1840

This PR also fixes read_all not including the other freedesktop appearance values, i.e. accent color and prefers color scheme.

[lenemter]

We don't need to add gsettings key for that. We should follow gnome's enable-animations key instead. I guess you tried to mimic what Gnome does there but gnome has separate options to reduce and to disable animation, so this doesn't affect us (at least not yet). But we difinitelly should look into this during OS 9 development cycle

[leolost2605]

@lenemter I'm not quite sure what you're saying. Right now there is the gnome enable animations key that both we and gnome use to completely disable animations. This is useful for performance e.g. in a VM but was misused as an accessibility setting. Therefore recently gnome introduced along with the free desktop portal spec a new key for reduce animation i.e. to keep some animations or alter them so that they are not straining or irritating for more people. They added a new key to their gsettings (https://gitlab.gnome.org/GNOME/gsettings-desktop-schemas/-/merge_requests/114) I'm not sure is that the key they you meant we should use? if so I agree that's a possibility but I think since the implementation is private to GNOME we should probably use our own key instead of relying on theirs. If you mean we should continue using the old enable animations key I disagree since these are separate things (performance vs accessibility). This of course means we should switch the settings toggle for reduce motion to this new key and maybe migrate existing users.