Make MessageDelegateBase::cleanupIncubator static

This doesn't need to be attached to an instance of MessageDelegateBase,
and avoid various problems when called from a MessageObjectIncubator
callback. (Because it needs access to the "this" pointer.)

Fixes a regularly occurring crash when switching rooms.

src/timeline/messagedelegate.h

@@ -222,7 +222,7 @@ private:
         }
         cleanupIncubator(incubator);
     };
-    void cleanupIncubator(MessageObjectIncubator *incubator);
+    static void cleanupIncubator(MessageObjectIncubator *incubator);
     void cleanupItem(QQuickItem *item);
 
     qreal m_spacing = 0.0;