The URL for the avatar depends on whether the call participant is a user
or a guest and, if it is a guest, on its nick. Although the user id of a
participant does not change if the participant is a guest the nick may
be changed during a call, so the avatar URL needs to be updated as well.
As the avatar URL is fully derived from other properties it is now
calculated internally in the ParticipantDisplayItem and calculated when
any of the properties it depends on changes (also for the user id for
completeness, as technically the item could be reused for a different
participant with a different user id, even if it is not currently done).
Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>
