Add PDDL as data license and support old CC licenses

[nuest]

PDDL is a common license for data, but it is not in the list of available licenses: https://opendatacommons.org/licenses/pddl/

Also, old CC licenses seem not to be in the list, although it can be expected that existing material is published under these licenses, e.g. CC-BY-3.0.

In this case the editor should simply hold (and save) the text given in a licenses property in the header.

Related to #184