Delete draft changes and patchsets

Adds ability to delete draft changes and patchsets that are not meant
or fit for code review. Deleting a draft patchset also deletes the
corresponding ref from the repository and decrements the next patch
set number for the change if necessary. Deleting a draft change
deletes all of its (draft) patchsets.

Change-Id: I04abeb67b64dd2366514e74d23f83066d409904e

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

@@ -140,6 +140,9 @@ public interface ChangeConstants extends Constants {
 
   String buttonPublishPatchSet();
 
+  String buttonDeleteDraftChange();
+  String buttonDeleteDraftPatchSet();
+
   String pagedChangeListPrev();
   String pagedChangeListNext();