Types and Keys of EntityFields are not saved on export

Right now when exporting the diagram or reloading the Apollon editor, only the name property of EntityFields are saved. The field's type and keys are lost.

This is because Apollon expects each UMLElement to looks at one user-visible value ("name") when serializing. Fixing this involves modifying the root interface that all elements implement, and would probably require a lot of refactoring.

A better solution in this case would be to make a field a container, then add three separate elements for the name, type, and keys. I will try to get this done next sprint.