(edit_get_byte): refactoring: use edit_get_byte_ptr().

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(edit_insert_column_of_text_from_file): refactoring:

  * move from src/editor/editcmd.c into src/editor/edit.c and make static;
  * rename to edit_insert_column_from_file;
  * make some refactoring.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(edit_insert_over): move from src/editor/editcmd.c into src/editor/edit.c.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>