Provides sample plugin that sets Code-Review+1 on entering LGTM into
reply dialog.
Adds a getLabelValue(labelName) to plugin.changeReply().
Adds support for plugin dom hooks in general, and for reply dialog text
area as a first instance.
Adds plugin.changeReply().addReplyTextChangedCallback() which uses dom
plugin hook.
Feature: Issue 6280
Change-Id: I2b8d52c0d8000ea5d217268f6e6d7ef4137b2213
7d23f225f7