make defaults for groupBy configurable

We do not know which fields exist at compile time.
But it is a great help to have some pre-selected
fields in groupBy.
Solved by adding a configuration option.

pdb-ui/src/main/java/org/lucares/pdbui/PropertyKeys.java

@@ -18,5 +18,7 @@ public interface PropertyKeys {
 	 */
 	String PRODUCTION_MODE = "mode.production";
 	
-	String QUERY_EXAMPLES = "query.examples";
+	String DEFAULTS_QUERY_EXAMPLES = "defaults.query.examples";
+	
+	String DEFAULTS_GROUP_BY = "defaults.groupBy";
 }