implement an undo/redo history.
- automatically do limited merging of consecutive edits - keystrokes for merged to the token-level, that sort of thing. Will contain all data necessary and not actually touch the clipboard, everything being reduced to "insert text" or "remove text."
- save the history to a scratch file, with header info identifying the source file.
- on opening file, check for pre-existing scratch file and load them to recover unsaved changes
implement an undo/redo history.