GtkApplication: Handle DESKTOP_AUTOSTART_ID leaked from parent

When this happens, our own register_session=TRUE had no effect. This will potentially cause loss of unsaved work on logout/shutdown, depending on how the application was started. This issue is rather hard to debug, because the old error message is not very hinting. Doing a google search on 'org.gnome.SessionManager.AlreadyRegistered' shows lots of such errors and in general users don't understand and ignore it.

While the bug is actually in starting application, the consequences are significant enough to justify a workaround.