window-list: Honor changes in skip-taskbar property

Although window-list checks the 'skip-taskbar' property when a
window is added to the desktop to decide wether it should be
shown in the bar or not, it doesn't honor that when the property
is changed after a window has already been added. Since the new
WaylandClient API allows to change this property for already
mapped windows, supporting this is a good idea.

This patch fixes this.

Fix https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/130


(cherry picked from commit b65f362f)