Add DC_EVENT_SELFAVATAR_CHANGED (#2742)

* Add DC_EVENT_SELFAVATAR_CHANGED

* Add test.

Unfortunately I can't easily also test that the avatar is not copied
from unencrypted messages:

In the second encrypted message, the avatar would not be sent again
then, because we only send avatars once a day or so.

* Unfortunately I can't easily also test that the avatar is not copied from unencrypted messages:

In the second encrypted message, the avatar would not be sent again
then, because we only send avatars once a day or so.

src/events.rs

@@ -323,4 +323,7 @@ pub enum EventType {
     /// dc_get_connectivity_html() for details.
     #[strum(props(id = "2100"))]
     ConnectivityChanged,
+
+    #[strum(props(id = "2110"))]
+    SelfavatarChanged,
 }