Make EnumerationProperty more flexible

Summary:
Reformat code

Store enumeration values in class instances

Also, change documentation to make clear one has to derive from
this class to really use it and to be able to serialize it.

Test Plan: I tested manually and wrote/read scene files with enum properties.

Reviewers: goch

Reviewed By: goch

Maniphest Tasks: T24541

Differential Revision: https://phabricator.mitk.org/D110