Overhaul of Insight's Search, Part 2 (rebased onto develop)


This is the same as gh-2676 but rebased onto develop.

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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.