The aggregate menu with its submenus isn't well-suited for simple
on-off actions, so we didn't expose the global color-scheme support
that was introduced last cycle.
Quick settings on the other hand are a natural fit for actions like
this, so add a corresponding toggle.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2392>
- move all icons to the icons directory
- rename some icons to be more meaningful
- put all icons on a resource sheet
- update references to icon name changes
- deprecate icons for those in standard set
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2141>