window-list: Set visibility based on the fullscreen status of 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 #400 (closed)