Code improvement - ExportForms

[ggalmazor]

#603 introduced some changes that highlight the need for refactoring ExportForms.load() to make it more concise and improve semantics.

There's also the question about what should BriefcasePreferences.hasKey() return in case the key exists but the value is null or an empty string. My guts say that we should return false, since we don't really know how the Java Preferences system handles removing keys, for example.