New UndoManager.

Basic implementation of an undo manager.  Supports
multi-level undo/redo, building on the top undo state
as edits occur, managing multiple distinct entities in
the undo state (such as embedded objects in a document),
and saving/restoring the full undo state.  Still some
work remaining on correctly dealing with dependencies
between undo states that hold multiple owners.

Also do a simple implementation of undo state in TextView
to see how things actually work.  The implementation here
is very primitive: it needs a lot more work to correctly
identify when to merge undo ops, is not trying to do
anything smart with style spans, etc.

Change-Id: Ie30f4e133351e2f569ffb48c6c44a2b19cadee27

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