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/config.rs

@@ -279,13 +279,13 @@ impl Context {
                         let mut blob = BlobObject::new_from_path(self, value.as_ref()).await?;
                         blob.recode_to_avatar_size(self).await?;
                         self.sql.set_raw_config(key, Some(blob.as_name())).await?;
-                        Ok(())
                     }
                     None => {
                         self.sql.set_raw_config(key, None).await?;
-                        Ok(())
                     }
                 }
+                self.emit_event(EventType::SelfavatarChanged);
+                Ok(())
             }
             Config::Selfstatus => {
                 let def = stock_str::status_line(self).await;