Unify API of lesson period with event and extra lesson by adding properties for group names

To be able to use the same API on the models lesson period, event and extra lesson which are often used in a common context (e. g. in Alsijil), I added the properties group_names and group_short_names to the lesson period model. They will use the connected lesson object for getting the real data.

added part::backend type::refactoring labels

merged

mentioned in commit e13bc720