suggestion: derivequeries panel should not delete pinned queries

I implemented this feature in our fork that might be relevant to others: pinned queries are not cleared if you generate new queries with the derivequeries panel, only unpinned are cleared. 

This is a first step in a feature proposal to manage the queries using tags (instead of names and pin boolean) : each query can have multiple tags. Panels can subscribe to queries using a tag list. A derivequeries panel controls one tag: it update/expand the queries by that tag, but when a query is already tagged by something else, a copy-on-write happens.