Rework how messages and events are handled

- No more translatable, content_type, show_name_line attrs for
  TimelineEventReceived.
  Since they are UI concerns, they are handled directly in QML.

- Refactor the EventDelegate and get rid of errors when new items
  are added to the timeline

- Messages, events and emotes all combine correctly.

- No more 28px wide avatars for events, to make them uniform with
  messages.

src/qml/EventHandlers/rooms.js

@@ -71,14 +71,12 @@ function onRoomForgotten(userId, roomId) {
 
 
 function onTimelineEventReceived(
-    eventType, roomId, eventId, senderId, date, content,
-    contentType, isLocalEcho, showNameLine, translatable, targetUserId
+    eventType, roomId, eventId, senderId, date, content, isLocalEcho,
+    targetUserId
 ) {
     let item = {
         eventType: py.getattr(eventType, "__name__"),
-
-        roomId, eventId, senderId, date, content, contentType, isLocalEcho,
-        showNameLine, translatable, targetUserId
+        roomId, eventId, senderId, date, content, isLocalEcho, targetUserId
     }
 
     if (isLocalEcho) {