Refactor base edit/cursor logic into separate $f_FileBuffer_raw_edit().
Initial setup for supporting undo in FileBuffer.
Change undo journal to a byte-oriented stream.
Implementation of $f_FileBuffer_read_undo_entry().
Implement undo in FileBuffer.
Implement FileBuffer redo.
Set/clear buffer modified status when un/redone back to saved state.
Use tmpfile() to open undo journal.