Add tag support

[Apollo3zehn]

This solves the "in which folder to I sort my files" problem. In the long run we will have a large amount of catalogs and the folder concept will reach it limits. It would be nice to have tags to group catalogs into locations, projects, topics and other categories.

How does this work in combination with the new aliases?

Tagged catalogs still need a name. And sometimes (e.g. in case of the HLL), the name is ugly (e.g. an id) and we need a prettier one: the alias.

How does this work in combination with Nexus.Federation?

Aliases cannot be defined from within the sub Nexus to be displayed high in the hierarchy in the main Nexus because currently aliases must be absolute paths beginning with a forward slash.

But main Nexus could define a special plugin which provides all these aliases. Maybe via .json file or web page to let users configure it easily. But this is no important feature right now.

Tags, on the other hand, will work for the main Nexus as well. But a big problem with tags is that catalogs are being loaded lazily and so many catalogs will not appear when doing a tag search :-(

I currently don't know how to solve that (lazy loading vs tags which require eager loading). Maybe Nexus needs some kind of cache of catalogs seen so far which updates itself when trying to actually access that catalog.

[Apollo3zehn]

Maybe use that catalog cache in general to avoid catalog being loaded before their first access? But that requires the cache to be invalidated when something changes ... just an idea