Subfilter under selection tags by namespace

When cleaning up tags and namespaces, I often find myself doing the same search, varying only by tags in one namespace.

For (a made-up) example, suppose you're cleaning up the tag asuka. You don't yet know whether this is supposed to be "character:asuka soryuu langley" from the Evangelion series, some other character from a different series, or something completely different.

Cleaning this up may involve filtering the list by different tags in the series: namespace. To do that now, you have to remove and add a tag to the search each time.

For this, it would be useful to have a couple dropdown lists near the "selection tags":

• one listing all unique namespaces found among the search results
• another which shows all tags of that namespace found among the search results when you change the first list.
• (and selecting the second one adds that tag to the search, replacing any other tags of the same namespace in the search.

You can sort of do this using the search box above "selection tags", this just speeds up the case where you want to filter by a particular namespace over and over.

Start from the selection-tags controls and the search-result tag data described in the issue, locating the desktop UI entry point that owns them. Done means the UI can list namespaces from current results, show that namespace's tags, and replace the selected namespace tag in the search.