Tag manager

[Ninjani]

Problem

It's hard to keep in mind all the existing tags and tag structure when adding a new tag.

Solution

Tagging in gooseberry should open up a search window with a list of existing tags to select from and a way to add new ones. This is an easy way to also allow selecting multiple tags at once.

Additional context

Maybe some way to create tag hierarchies like in Obsidian with "/". (this should probably be a separate issue as it would need refactoring of the query system).