3aa49b6fec
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