ChangeScreen2: Add change_plugins element

Like change_actions define change_plugins as an element identifed by
id for plugins to install their own DOM nodes into.

A plugin rendering build status information from a could listen for
view updates with Gerit.on("showchange", f) to capture the ChangeInfo
and RevisionInfo being presented, query the build server, and add that
data into the DOM node.

Change-Id: Ie1cfb6bc25292d9f54b0db8f565a0bb4079d713d

gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen2.ui.xml

@@ -441,6 +441,7 @@ limitations under the License.
           </table>
           <hr/>
           <c:Labels ui:field='labels' styleName='{style.labels}'/>
+          <div id='change_plugins'/>
         </td>
         <td class='{style.relatedColumn}'>
           <c:RelatedChanges ui:field='related'/>