Use SPDX IDs for marking licenses

[Matija Šuklje]

SPDX IDs and statements are the de facto standard for license names. It would make sense to use it here.

e.g. MIT, GPL-2.0-or-later or GPL-2.0 and LGPL-3.0 and Apache-2.0 are examples

[Cornelius Schumacher]

Yes, absolutely.