[On hold] Windowview: If no window matches the searchfield, present option to open new app

This replaces the "No Windows" placeholder with Krunner (the same as in the Overview effect) that takes its searchstring from the filter searchbar at the top. That means, if no windows match the search, KRunner will be visible (using the same searchbox that is used for window filtering). This doesn't interrupt the normal usage because it is only shown at a point where a placeholder message would have otherwise been shown.

The reason behind this is that I sometimes found myself searching for a window only to find out I had already closed it. At that point, I had to exit the effect and open an application launcher (and type the app name again). It would be more convenient to be presented with KRunner if there were no open windows left (because if there are no windows that match your search, you are probably looking for an app that is currently closed).

This experience is really seemless (try it out for yourself): You search for something and you get it. You don't have to think about if it's already opened or not, so you can just blindly type the name and press enter.