dbusServices/extensions: Stop stacking frames

dbusServices/extensions: Stop stacking frames

We currently use separate frames for the details expander and the
expanded details. That layout works as long as frames are boxy (as
in the default GTK3 style), but breaks down with rounded corners
(as in the default GTK4 style).

In order to work with either style, adapt the layout to use a single
surrounding frame and appropriate borders as separator.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1495>