status/system: Port to quick settings

status/system: Port to quick settings

This port is a bit messier than the previous ones, because the
existing menu section translates less directly to the new UI,
which uses a row of individual toggles for settings, lock and
shutdown.

In order to not complicate the grid layout further by supporting
rows with a different number of columns than the overall grid and
children at their natural size, create a custom, non-reactive
SystemItem item that spans an entire row, and contains the individual
toggles.

This works quite well, even with the shutdown item that uses a menu
for the various actions.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2393>