Initial work on BufferEditor.
Integrate BufferEditor into main, and fix minor issues.
Make $f_FileBuffer_edit() more resilient to invalid input.
Destroy BufferLineEditors if the line they are editing is changed externally.
Destroy BufferLineEditor on abort.
Omit $H symbols from doccheck.
Get some basic commands of BufferEditor working.
Don't seek in $p_shared_undo_log.
Update workspace echo area when Activities are created.
Suppress Activity echo area method when being destroyed.
Implement $f_BufferEditor_self_insert().
Fix order of echo area meta in BufferEditor.
Add temporary workaround for object_clone() not really doing what we want.