Add Summary Section to AccountFolderList ("at a glance")

* Wrap group cursor in a MergeCursor to insert 1st row
* Clean up a few loose ends (e.g. no longpress on summary row)
* Implement the summaries child cursor
* Add special queries to MessageList for Inbox, Unread, Favorites

There are *a lot* of TODO's in this checkin;  I just wanted to get some
basic UI up and running so we could start to explore issues created by
the new configuration.  A lightweight sampling:
* Aggregrate filters (e.g. all inboxes) need to be handled in the
provider, instead of with the current two-query hack.
* We'll need a lot of UI cleanup, e.g. different contextual menus
* We'll need to generalize some of the code in order to more easily
support repeating the same compound queries - e.g. MessageView actually
needs the same query in order to enable the next/prev buttons.
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