Let user import/export custom datatypes

Users should be able to import/export their own list of custom datatypes. In that way, we could define the “language” used within a diagram. There should maybe be an option to choose the language in the beginning or set it via the application settings. Should be checked what’s easier.

We could add an option that lets users choose to convert all existing classes as standard types, so they could easily be reused in a new diagram.