Overhaul of Insight's Search, Part 2

Part 2: Displaying the search result in a table view. 
(I branched this off my PR #2668 'Overhaul of Insight's Search, Part 1')

Does not really work yet. The results are shown, but the thumbnails aren't loaded and the "View" button only works if all projects and datasets of all groups are already expanded.

Maybe this should be marked as on-hold for now.

maybe a `SetMultimap` would be useful