Add ability to selectively remove styles from source files during merge

[yonkoma]

No description provided.

[Myaamori]

I've been meaning to add an option to remove unused styles, which I think could be combined with this change. E.g. there could be an enum similar to OnNegativeTimestamp where you can choose to either leave styles as is, remove unused styles, or remove all styles.

The question, though, is whether to keep it at the top level or in the merge specification. For removing unused styles, it's likely that the top level is more appropriate, while when removing all styles, you probably want to do it for individual files. Would you mind describing what your use case is, exactly? Is it possible that just removing all unused styles would satisfy it?

I was planning on implementing removing unused styles myself at some point, but if you're interested in looking at it for this PR, it should be easy enough by filtering dialogue lines to select uncommented ones, mapping to the name of the style, and then using toSet to get a set of used styles, after which you can remove all styles not present in that set.

P.S. It would be better if you refrained from updating the documentation with this pull request, as that will break all links unless I change the current version tag.

[yonkoma]

The use case for which I did this is editing an existing subtitle file, with styles that we aren't using. We have the styles in a separate common file, and have some issues with conflicting styles. I did set style conflicts to fail, and also make a local task to achieve this, but this would be easier and cleaner.
As far as I can tell the use cases are separate, and both are worth implementing.