[windowTracker] Only remove "interesting" windows

Windows are only added to an application if they are considered
"interesting". If we keep it that way, we cannot unconditionally
call _shell_app_remove_window() - applications without interesting
windows are not considered running, so the call crashes the shell.

https://bugzilla.gnome.org/show_bug.cgi?id=622236

src/shell-window-tracker.c

@@ -542,7 +542,8 @@ disassociate_window (ShellWindowTracker   *self,
 
   g_hash_table_remove (self->window_to_app, window);
 
-  _shell_app_remove_window (app, window);
+  if (shell_window_tracker_is_window_interesting (window))
+      _shell_app_remove_window (app, window);
 
   g_signal_emit (self, signals[TRACKED_WINDOWS_CHANGED], 0);