Add a simple annotation layer that marks trailing whitespace in diffs
(guarded by the `show_whitespace_errors` diff preference). The newly
supported diff preference is added to both diff preference controls. The
requirement that all annotation layers must implement `addListener` is
relaxed as the trailing whitespace layer is the third layer that doesn't
use it.
Adds tests for the layer and the diff preference.
Feature: Issue 4836
Change-Id: Ifba05216bf0bc3c0a8a094f5ef392b983091d59f
d970500d62