Dark style API + HC rework

See https://twitter.com/alexm_gnome/status/1430522927141826560 for context.

Draft until at least gsettings-desktop-schemas, xdg-desktop-portal and xdg-desktop-portal-gnome (and probably -gtk too) parts have landed.

Opening a MR so there's something tangible to discuss.

Fixes #52 (closed) #54 (closed)

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Teams/Design/settings-mockups#27 (closed)

• libhandy!782 (merged)
• gsettings-desktop-schemas!51 (merged)
• xdg-desktop-portal-gnome!12 (merged)
• https://github.com/flatpak/xdg-desktop-portal/issues/629 (fixed)