The overview is using a heavily darkened background image nowadays.
Since the contrast provided by this is always good enough, this makes it
possible to always have a fully transparent panel while showing the
overview.
So change the theme to always make the panel fully transparent while the
overview is shown. The duration of the animation for that is 250ms
because that's the duration of the show and hide animation of the
overview.
Also add special handling for the high-contrast theme, where we always
make the panel opaque.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1397>
- panel fg color was derived from bg color. Not a good idea as it's not
just light/drk, but HC as well.
- deriving from dark theme means contrast for things like popover items
is better.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/904
The sass sources now live in a project in GNOME, so they can
be used in multiple projects like gnome-shell-extensions.
Because of that, add gnome-shell-sass as a submodule of gnome-shell
and remove the sass sources from the theme directory.
