Sort rules first on group

app/src/main/java/eu/faircode/email/AdapterRule.java

@@ -650,10 +650,15 @@ public class AdapterRule extends RecyclerView.Adapter<AdapterRule.ViewHolder> {
                     order = Integer.compare(r1.order, r2.order);
 
                 if (order == 0)
-                    return collator.compare(
+                    order = collator.compare(
+                            r1.group == null ? "" : r1.group,
+                            r2.group == null ? "" : r2.group);
+
+                if (order == 0)
+                    order = collator.compare(
                             r1.name == null ? "" : r1.name,
                             r2.name == null ? "" : r2.name);
-                else
+
                 return order;
             }
         });