KDE always had single click file opening/launching/etc - even from KDE1 days. In old KDEs -IIRC- you select stuff by hovering them for a second or so, in new KDEs when you hover an icon there is a "+" sign over it and if you click it, it adds the file to the selection (and becomes a "-" sign). You can also simply Ctrl+Click or Shift+Click on things to select them, only plain click opens/launches a file.