Evaluate setting for styles based on other styles

[dariok]

Styles can be based on other styles (e. g. a custom heading can be based on Heading 1).
In these cases, only the differing settings are stored in the style definition while others have to be copied from the original style (worst case: cascading...). As this may involve outline level info, we need to evaluate and combine these settings before we write them to tei:rendition (later override earlier).