OPENDJ-1714 Code cleanup

* Installer.java
** Minor refactorings in method updateUserDataForNewSuffixOptionsPanel(QuickSetup)
*** Fix minor UI bug with invalid/valid fields
*** Reformat
*** Add final keyword
*** Replace hardcoded line separator by constant
*** Extracted methods:
**** checkProvidedBaseDn(QuickSetup, List<String>, List<LocalizableMessage>)
**** checkImportLDIFFile(QuickSetup, List, boolean, List)
**** checkImportGeneratedData(QuickSetup, List, boolean, List)
**** checkImportData(QuickSetup, List, boolean, List)