shell-app: Track changes to MetaWindow:skip-taskbar

So far we have assumed that whether or not a window is interesting
is static. In general this is the case, but as it is legal for the
underlying properties to change at any time, there are of course
offenders that actually do this (flash I'm looking at ya).
While we used the property to determine whether a window should be
tracked or not, the worst case was showing windows that should be
hidden or missing windows that should be shown.
However as we nowadays base an app's running state on the number of
interesting windows, we need to be more careful in order to avoid
ending up with running apps with no windows.

https://bugzilla.gnome.org/show_bug.cgi?id=723308