Added accessor method 'isUndefined()' to several value objects
Renamed field 'content' to 'value' and added accessor method
Added constant annotation for '@Nullable' and is-methods
Renamed accessor method 'Type.getTypeName' to convenient 'Type.getName'
Changed type of field 'imports' to arbitrary list of 'Import'
Added constant annotation for '@NotThreadSafe' and is-method for it
Added field 'accessorPrefix' to Field class to a possible determine
accessor method for it and revised depending classes