Eagerly update when turning the mode on.

This is an attempt to reduce flickering when showing a new window.
Use `line-beginning-position` instead of `point-at-bol`.

The latter is a XEmacs compatibility alias for the former.
In `relative-line-numbers--scroll`, select the window so the scheduled update runs in it.
In `relative-line-numbers--set-margin-width`, `and` is more idiomatic than `when`.

Version 0.2.2