merge plasma-integration:059369d6b6a9a99538c175a3c53058cbb54e8abc partly and fix some compiler warnings.

I don't want to introduce a QtQuickControls2 dependency, possibly
initialising that component in *ALL* applications, so I'm sticking with
the env. variable approach. No plasmashell or krunner on Mac anyway.

Committed from host : Portia.local