Bugfixes and singular field values

Fixed a bug that encrypted fields in transfer settings were encrypted or
decrypted multiple times. This resulted in the link which was generated
while init being encrypted twice.

Added singular field parameter to setup annotation. Singular fields have
to be entered every time a value is queried. For instance, if a user
revises setup values during init singular fields are not cached. This is
needed for plugins which do oauth since access tokens might be
invalidated after the first call (this is case with the dropbox plugin).
Use custom action aware encryption converters

Previously all transfer settings values were encrypted once they are
persisted independent from the location. Now only configs written to
disk are encrypted using a custom converter.

Changed default @Encrypted handling and removed persistence callbacks in
transfer settings.

Deprecated generic AbtractInitOperation#writeXmlObject. Every *TO
should use its own save method because the above case has shown that
persistence isn't generic in all cases.