Disable() should restore the scheme preference that was used when
the extension was enabled, not when it was first initialized.
Even if it's unlikely to be relevant in practice, let's make sure
we save the correct state.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/257>
Now that gnome-shell supports a light style, people may want to
use it without forcing all apps to be light.
Add a small extension that switches the default to light, so the
shell follows the regular "dark style" preference.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/256>