We no longer need to restore selection ranges before and after
transactions now because selections are based on markers so they go
along for the ride for free. This allows us to delegate directly to
Buffer.transact from EditSession.
When `UndoManager.transact` is called with no function, you later need
to `abort` or `commit` the transaction manually. This allows
transactions to last longer than the dynamic scope of the single
function passed to `transact`.
The `transact` method takes a function and batches all operations within that function as a single transaction to be undone and redone. The edit session now uses generic operations to restore selection state around transactions. The `undo` and `do` methods on operations are now optional. In addition, the undo manager now supports an optional `redo` method on an operation for code that should *only* be run on redo, and not when the operation is initially pushed. This is used by edit session to restore selection state after redo.
This only restores changes made via the CompositeSelection… this makes sense because this is the only way to make changes interactively. Any other changes are made via the api or a command line and should not modify selections when they are undone. Still need to test restoration after redo.
