window-list: Fix initial visibility

Mutter uses an undefined initial in-fullscreen state, so it will
always emit the `in-fullscreen-changed` signal when it determines
the actual initial state.

This didn't use to be an issue when the shell started in the session,
but now results in the window list ending up visible in the overview
on startup.

Work around this by hiding ourselves again when the in-fullscreen
state changes in the overview.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/185>