mceditor: refactoring.
* (mcedit_arg_t): rename to edit_arg_t and move to src/editor/edit.h.
* (mcedit_arg_free): rename to edit_arg_free() and move to src/editor/edit.c.
* (mcedit_arg_vpath_new): rename to edit_arg_vpath_new(), move to src/editor/edit.c
and make public.
* (mcedit_arg_new): rename to edit_arg_new(), move to src/editor/edit.c
and make public.
* src/editor/editwidget.c: sync with new editor APIs.
* src/main.c: likewise.
* src/args.c: likewise.
* (mc_args_parse): show error in case of run MC as "mc -e" if MC is built
without builtin editor.
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>