Award a Kudo if an application has perfect screenshots

'Perfect' in this sense means with a sensible size range, and 16:9 aspect.
Fix a small memory leak in the AppStream plugin
Automatically suggest new application picks based on a heuristic

If you've got an installed application in a subcategory, suggest other desktop
applications from that same subcategory if they are not-installed, have perfect
screenshots and enough kudo points.