Make tags hierarchical

[matthijskooijman]

Allowing to define tags in a hierarchical way could be helpful, to:

• Categorize different tags, e.g. split top-level categories ("Sports", "Politics"), from recurring rubrics ("Grimlachjes") from in-depth dossiers ("Westelijke ontsluiting"), from internal tags (i.e. "Featured").
• Within top-level categories, allow subcategories (i.e. "Sustainability" with subcategories "Solar energie", "Waste reduction", etc.).

Such a hierarchical structure is mostly useful for displaying structure within tag lists and tag pages to the viewers. They can also be used to show an indented view when adding new tags to pages. I'm not sure if they should be displayed in the tag list when viewing a page, maybe it could affect sorting and CSS classes so CSS could add some filtering and/or structuring of these?