Fixes "Can't assign to existing role 'subitems' of different type"
or hang (depending on Qt version) on using nested subitems.
Keep nested subitems flattenend as json string in memory while not
being used, to simplify dealing with ListElement expectation that
all elements have the same data type, as well as QML/Ecmascript's
memory management.
Floris Bos
c17795c48e