Rework compare to take into account closed windows

Rework compare to take into account closed windows

The API docs for ShellApp claimed it sorted by the last time the
user interacted with the app, but if one closed a window, then
we would fall back to comparing against a possibly much older
timestamp from another window.  Fix this by just keeping a
user time per app.

Also clean up the comparison function to explicitly check the state
instead of deferring to the window list.

https://bugzilla.gnome.org/show_bug.cgi?id=618378