Reduce opacity of avatars for peers that are not connected

Until now the opacity of the avatar was reduced when the peer was
created until it got connected for the first time. Now the opacity of
the avatar is reduced whenever the peer is not connected. In practice,
the difference is that now the opacity is changed too when there are
issues with the connection to the other peer.

Note that the issues were already signaled using the loading icon, but
only after a long disconnection; now the opacity is reduced as soon as
there is a disconnection, which provides quicker feedback of the issue
while keeping the previous behaviour (as the loading icon is still shown
in a long disconnection).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>