DocIndex: Edit target counts

[mkoskim]

This issue was earlier for Story Arc View. Now when containers (acts, chapters and scenes) have word count target attributes, we should be able to edit them in the story arc view. The whole idea to implement target counts were to allow story structuring in the arc view - editing fillers (previous way to pad the story) was bit too laborous.

But when thinking about the issue, it could be good to make the DocIndex itself sort of an editor. At the moment, DocIndex acts as an editor by letting you to reorganize text, but this could be extended:

• Edit target counts (what we want for Story Arc View)
• Edit filler counts (when we place them to index again)
• Edit other bookmark properties
• Edit name (could be good)
• Fold on/off
• Add new (empty) containers
• Remove containers (at least empty ones)

But anyways, it would be best to start with ability to edit container target word counts, as it would immediately make Story Arc View much better.