Firefox and LibreOffice aren't part of KDE, but Kate and Okular are. So the fact only the former two support this feature indicates it's not from KDE. This should probably be supported by KDE though (or by Qt? perhaps this should be done on the toolkit level rather than on the application level). Holding ctrl to multi-select is standard behaviour in a lot of applications and is quite useful. For what it's worth, Krita does support this, but you need to hold shift instead of ctrl.