Fixed two bugs with Multi Entity fields value() accepting 'all' and 'default'

Multi Entity fields value() did not properly set the names of fields to sync
when 'all' was specified.  It was using the field parents fieldNames()!

When 'default' was passed the code thought it was dealing with an Entity object
however the Entity object was not yet created and the data was a dictionary.
Fixed SgFieldID and SgFieldType never having valid set to True

When the class property for valid was moved to being private the ID and Type
field classes were not updated to reflect this change so they were never
properly setting themselves as being valid.

This didn't have any impact on code except it meant that isValid() would always
return False.