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/spacetreeitem.h

@@ -125,6 +125,11 @@ public:
      */
     bool isSpace() const;
 
+    /**
+     * @brief Return the m.space.child stripped state Json for the given child.
+     */
+    QJsonObject childState(const SpaceTreeItem *child) const;
+
     /**
      * @brief Return the m.space.child state event content for the given child.
      */