Fix avatar loading in multiple places and prefers name instead of

display name for avatar fallback.

This also fixes a bug where users didn't get their avatar loaded in the
room list.

Fix #209

src/roomlistmodel.h

@@ -39,6 +39,7 @@ class RoomListModel : public QAbstractListModel
 public:
     enum EventRoles {
         NameRole = Qt::UserRole + 1,
+        DisplayNameRole,
         AvatarRole,
         TopicRole,
         CategoryRole,