ShellWindowTracker: don't create ShellApps for non-interesting windows

ShellWindowTracker: don't create ShellApps for non-interesting windows

If a process does not have any "interesting" windows, then it can't be
considered a running app. (Previously we were calling
get_app_for_window() before ruling out non-interesting windows, which
ended up calling _shell_app_new_for_window(), which would add the
window to the ShellApp directly, bypassing the is_interesting check.)

https://bugzilla.gnome.org/show_bug.cgi?id=642221