session: Call update_info() if service changes

session: Call update_info() if service changes

Instead of trying to figure out when to call update_info() at various
places, just call update_info() at the end of session_changed when the
selected service has been changed.

Another noteworthy change is the remove of g_timeout_add_seconds(...,
session_notify(), ...). It was introduced to 'fix' a problem where
sometimes the notification message was not send. The source of this
problem seemed to be that session_notify() couldn't be called if the
current context was called from a incoming D-Bus call
(e.g. Session.Connect()). Though without g_timeout_add_seconds it
still works now. So it's properly better to remove it again since it's
seems not necessary.