window-list: Set visibility based on the target monitor.

Currently, the visibility of the window list on a given monitor is set
according to the fullscreen status of the primary monitor. When a
fullscreen application is on a secondary monitor (but not on the primary
monitor), entering and exiting the overview will lead to the window list
incorrectly being visible on the monitor with the fullscreen
application.

Instead, determine the visibility based on the fullscreen status of the
monitor being evaluated.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/400

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/230>