Remove ImageItem, Paintable and use unified image provider.

src/userlistmodel.h

@@ -17,7 +17,7 @@ class UserListModel : public QAbstractListModel {
   Q_PROPERTY(
       QMatrixClient::Room* room READ room WRITE setRoom NOTIFY roomChanged)
  public:
-  enum EventRoles { NameRole = Qt::UserRole + 1, UserIDRole, PaintableRole };
+  enum EventRoles { NameRole = Qt::UserRole + 1, UserIDRole, AvatarRole };
 
   using User = QMatrixClient::User;