Support descriptions for tags

[itsmvd]

It would be useful if, when adding a new tag to an event, you could add an optional description to provide additional context about the tag applied.

Ideally this is free format and just provided as an option via the API, i.e. add_tag(name, description), analyzers & other tools can then make use of this to provide additional context directly in the sketch.
The tag description would show up on hover over the tag.