shell-app: Base running state on "interesting" windows

An app should be considered running if it has at least one "interesting"
window, however the code considers an app running if it has at least
one tracked window. This was fine while we were only tracking interesting
windows, but since commit d21aa0d85fc325 this is no longer the case.
So keep track of the number of interesting windows as well and use that
to determine the running state.

https://bugzilla.gnome.org/show_bug.cgi?id=722690