Add KDE Gitlab Invent web search keywords

This adds two new web search keywords for Invent: one that uses full search (invent.desktop) and another that only searches for projects and sorts them by number of stars (invent_repo.desktop). The latter ensures that the first results will always be "official" projects (as opposed to personal repo projects), since those pretty much always have the most stars.

This should make searching through Invent much nicer and faster than using the web interface.