[SYNCOPE-2001] Fix for plainAttrs in readonly mode in customFormLayout (#1570)

Author: giusmad

client/idrepo/enduser/src/main/java/org/apache/syncope/client/enduser/panels/any/PlainAttrs.java

@@ -367,7 +367,7 @@ protected void populateItem(final ListItem<Attr> item) {
                     }
 
                     AbstractFieldPanel<?> panel = getFieldPanel(schema);
-                    panel.setReadOnly(schema.isReadonly());
+                    panel.setReadOnly(schema.isReadonly() || ((FieldPanel<?>) panel).isReadOnly());
                     if (schema.isMultivalue() && schema.getType() != AttrSchemaType.Dropdown) {
                         panel = new MultiFieldPanel.Builder<>(
                                 new PropertyModel<>(attr, "values")).build(