Preferences use internal defaults instead of consulting the default scope

As outlined in https://www.eclipse.org/forums/index.php/t/1114430/ for a specific case, SW designer only consults the instance scope for preferences. If no configured value is found there, an internal default value is used instead of doing a lookup in the default scope.

The issue is caused by the replacement of jface preferences to remove of UI-related dependencies (notably jface) from code generators (see eclipse bugzilla 580187)