Thanks Mathias and Jungle for your explanations and your videos, Jungle!
I have done as you suggested, but please let me try to explain what the missing part is:
I can do the one-click opening/activation alright, as in explorer.
But if I then try to select files by pressing the ctrl-key and left-clicking files, they should get selected (or unselected), but the one click opening does not allow that, instead opens the file or folder. I can then go back to the original folder and see the files/folders selected, but that is not as expected.
Hope I could make myself understood.
Thanks for your efforts.
Norbert