Add keyword "version"

As my first MR in this repo, I propose adding the keyword "version" to the main .desktop file, so that KInfoCenter comes up in KRunner and Kickoff when searching for it. Currently, nothing relevant comes up when typing said keyword, so I think that would be sensible.

By the way, I followed @niccolove's "tutorial" on how to add such keywords – thank you for making these videos! :)