Improve cached properties by reducing number of them

[hagenw]

The code can be made easier by using a few ground truth cached properties, from which then all other properties could be derived. So far we did not follow this approach to avoid storing large quantities in the cache (e.g. we don't need the whole header).
But this can be avoided, by first extracting all information, that we need and store them inside a dictionary and then access the dict entries in other properties, which we don't need to cache.

To make all needed properties available in Datacard, we need to add a new _properties() method and use those in Datacard, insteaf of _cached_properties().

[hagenw]

E.g. instead of having _segments, _tables_stats, tables_columns, tables_preview, tables_rows, segments, segment_durations as cached properties, we could replace it by a single _parse_tables cached property, which then returns a dict with entries for all the other properties that need table information.