clears both undo and redo buffers
returns operation to be redone (put it to undo), null if no undo ops available
adds undo operation
current state is saved
returns true if saved state is in redo buffer
returns operation to be undone (put it to redo), null if no undo ops available
returns true if buffer contains any redo items
returns true if buffer contains any undo items
returns true if content has been changed since last saved() or clear() call
Undo/Redo buffer