Tags: Selected tags to index items

[mkoskim]

Select tags that are shown in the index (scenes, chapters, acts). For this purpose, we might first need to speed up tags just like we did with word counts.

Reasoning: For example, you may have POV (Point of View) tags, and you want to see the POV in the index, probably at chapter level.

Note-to-myself: Tags need some kind of grouping feature.