[Oberon] Oberon keyboard tweaks

[Hans Klaver]

> But CTRL-C, -V and -X is "officially" supported in PO2013:
> https://people.inf.ethz.ch/wirth/ProjectOberon/Sources/TextFrames.Mod.txt

I suppose Jörg wrote "officially" because it is only weakly supported in PO2013; these ctrl-combinations only work within one and the same textframe, not between different textframes.

Otoh, the procedures of Peter De Wachter's Clipboard.Mod (CopySelection, CopyViewer and Paste) work from/to the OS and PO2013, and within PO2013 also between different TextFrames. Linking these to ^C, ^X and ^V is difficult (at least, I didn't manage to get it working) trying to do so leads to runtime errors in sdl-clipboard.c

Regards,

Hans