In order to use an EOPM on top of the favorites model, we need
the config key to be fixed (e.g. "0") rather than "the parent
collection" in ETM, since the favorites are a flat list of individually
picked collection, with a different parent in ETM. We want a single
order. So the new virtual parentCollection is reimplemented in a new
proxy derived class in mailcommon, for ordering favorites.

With the mailcommon and kmail patches on top of this,
I can reorder favorites using drag-n-drop and the order is persisted.