SideBySide2: Add preference for a subset of supported CM3 themes

Add support for CM3 themes. CM3 has built in support for themes.
Unfortunately we can not use all of them out of the box, because
we set statically background colors on diff styles.  However we
can use light themes. Especially "eclipse" theme looks very sexy
for Java developers that use this IDE and are acquainted with its
look and feel.

Integrate all supported themes in Buck tool chain, so that we are
prepared to extend the support for dark themes. To do that we must
dynamically assign background colors for different diff styles per
theme.

Change-Id: Ic97c237d789dc4daeff6b8bb4f7606da83f5d532

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PreferencesBox.ui.xml

@@ -161,6 +161,10 @@ limitations under the License.
     </table>
     <hr/>
     <table class='{style.table}'>
+      <tr>
+        <th><ui:msg>Theme</ui:msg></th>
+        <td><g:ListBox ui:field='theme'/></td>
+      </tr>
       <tr>
         <th><ui:msg>Ignore Whitespace</ui:msg></th>
         <td><g:ListBox ui:field='ignoreWhitespace'/></td>