OPENDJ-2092 OPENDJ-1759 New Schema elements GUI

CR-7193
This commit fixes control panel "new attribute type" and "new object class" features.
Changes made for both features:
* src/main/java/org/opends/guitools/controlpanel/task/NewSchemaElementsTask.java
** Re introduce code to create attribute type or object class string definition. This is needed because at the moment we add schema elements with their definition. Since r10478, definition is no more computed on the fly on a toString() call. We need to duplicate code as the only way to get a schema element definition is to add it to a schema before. Once the schema support will be fully migrated to the sdk this duplicated code should be remove. Note that the description generated is used only by machines, not humans
* src/main/java/org/opends/guitools/controlpanel/datamodel/ServerDescriptor.java
** Fix regression introduced in r11261 which has side effect to avoid schema element descriptor refresh after a schema element creation.