Cross-project blueprint references

When referencing a blueprint from another one (for linking a dependency or superseding), it is often useful to to reference a blueprint from a project other than the one the first blueprint is registered in. A new huge vocabulary will be added, for use with the blueprint selection pop-ups in the relevant interfaces, which allows, apart from searching for the blueprint's name, title and summary, referencing a blueprint from any project in launchpad, by entering the blueprint's url, or any unambiguous part ot it.