effects/overview: ctrl + j/k search result navigation

KRunner and Application Launcher support Ctrl+J and Ctrl+K to navigate KRunner search results. This merge request adds the same capability to the overview search results.

This is !4039 (closed) retargeted to the master branch. But I've only compiled and tested on Plasma/5.27 because that's what kdesrc-build built for me. Sorry, I'm new here :)