Implement web interface for patches diff

Support diff between patches in the change screen. Allow changing
patches base comparison.

All displayed patches can now have their files listed and compared
to target branch or to a specific patch of the change.

Bug issue: 194
Change-Id: I617cbe647338b8a4fb71788f04ab6e2c952780fc

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeConstants.java

@@ -107,6 +107,8 @@ public interface ChangeConstants extends Constants {
   String buttonAbandonChangeCancel();
   String headingAbandonMessage();
   String abandonChangeTitle();
+  String oldVersionHistory();
+  String baseDiffItem();
 
   String buttonReview();
   String buttonPublishCommentsSend();