Updating an existing notification in the SDK guidance service will
result in a D-Bus call to the notification daemon. Relying on
GObject:notify signal means we'll have to do one D-Bus call per change
(title, body, icon) while using this new signal allow us to update the
whole notification in on shot.

Signed-off-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Reviewed-by: Justin Kim <justin.kim@collabora.com>
Differential Revision: https://phabricator.apertis.org/D5362