Display tag and categories as tree

From here https://github.com/eikek/docspell/issues/568#issuecomment-761533086 (and following comments)

This is only about the view of the category and tags. Currently the list of categories is rendered below a list of tags, where both are sorted by must usage. Another option is to provide a nested tree-like view.

This is probably also required for nested tags, if #413 is done first, then this issue is void.