Fix BufferLineEditor inheriting the BufferEditor's keymap.
Bind C-O to $f_BufferEditor_insert_blank_line_below.
Update the echo area when the BufferLineEditor is opened.
Have Activity null its keymap on construction.
Fix subclass() not implanting parent this.
When accepting an edited line, remove window change notification hook first.
Implement $f_BufferEditor_forward_line() and $f_BufferEditor_backward_line().