Support the order parameter in m.space.child events

Support the order parameter in m.space.child events, with the exception of always putting spaces higher than non-spaces

see https://spec.matrix.org/v1.8/client-server-api/#ordering-of-children-within-a-space for the spec algorithm

src/models/spacechildrenmodel.h

@@ -51,6 +51,8 @@ public:
         IsDeclaredParentRole,
         CanRemove,
         ParentRoomRole,
+        OrderRole,
+        ChildTimestampRole,
     };
 
     explicit SpaceChildrenModel(QObject *parent = nullptr);