New indenting commands

- rename `indent_size` window field as `indent_width` for consistency
- handle negative argval in `do_tabulate`
- fix reverse region in `indent-rigidly`
- only indent lines that start in the region, ie: lines whose beginning
  offset >= start and < end
- fix `do_backspace` to delete a TAB worth of white space after TAB.
  no longer use `undo` for this.
- fix `find_window` to locate window according to top-left corner of current
- add `unindent-region` on `M-C-|` to unindent current region
- add `unindent-buffer`
- redirect `S-RET` and `S-DEL` to their unshifted versions
- rename `c-indent-command` command as `c-indent-line-or-region`

lang/python.c

@@ -342,7 +342,7 @@ static int bazel_mode_init(EditState *s, EditBuffer *b, int flags)
     if (s) {
         /* XXX: should use the default values from mode variables */
         s->indent_tabs_mode = 0;
-        s->indent_size = 2;
+        s->indent_width = 2;
     }
     return 0;
 }