make s-c-p optional according to upstream comments

...from https://bugs.kde.org/show_bug.cgi?id=320326

I chose to keep the dependency behind the 'gtk' flag for lack of inspiration, enabled by default:
- desktop profile users will get it as upstream intended, after all it's 'important'
- if the user has consciously disabled 'gtk', there is a fat ewarn message explaining state of affairs