Tags: Replace tag categories with actual (root) tags

[riipah]

Tag categories would be replaced wags with no parents, called "root tags".

Possibly a checkbox that would mark the tag as "container tag" that prevents it from being used for tagging. This option could be used for middle levels as well (is there use for this?)

Benefits:

• Automatically see children of the same root tag (siblings)
• Root tags can be translated
• Description can be provided

Open issues:
Categories are only one level, which is actually good, because often you want to see the root category. For example genres - it's important that the user can easily see that the tag represents a genre.

It would be possible to dig the root tag for each tag at runtime, but this would be slower. It's better to maintain the root tag for each tag when the tag is moved, even if this requires additional work.