OPENDJ-1932 Code cleanup

* NewBaseDNPanel.java
** refactor okClicked()
*** Reformat
*** Add final keyword
*** Extract local NEW_BACKEND_TEXT constant
*** Extracted methods
**** resetLabelAsValid()
**** validateBackendName(final Set<BackendDescriptor> existingBackends, final Set<LocalizableMessage> errors)
**** validateBaseDN(final String backendName, final Set<BackendDescriptor> existingBackends, final Set<LocalizableMessage> errors)
**** validateImportLDIFFilePath(final Set<LocalizableMessage> errors)
**** validateAutomaticallyGenerated(final Set<LocalizableMessage> errors)