CORE, GUI: proper checking for pending mail validation

CORE:
- Do not return pending mail change request for already
  validated emails (if value is set in attribute).

GUI:
- Proper implementation of check on pending preferred
  email change requests. User can see, what email addresses
  has change pending.
- Removed usage of LocalStorage for pending mail change.
- BasicOverlayType can now return list of Strings, Booleans
  and Integers from JSON response.