This optimizes the sources model by avoiding model resets and cleaning up the code.

KRunner feels a lot snappier. It does not fix the initial lag we get when ListView creates all the delegates but subsequent updates to the results should be noticeably faster. Unfortunately KRunner jumps around a bit sometimes when results come and go which was previously probably covered by it just blocking without rendering that.

I didn't understand the way the results are stored in the model thorougly enough to make it a proper model with beginInsertRows/endInsertRows when new results are added, though.