KColorScheme: Add FrameContrast API

Akseli Lahtinenrequested to merge
work/akselmo/update-contrastF into master

contrastF is very overloaded and used by many, this frameContrast value is used by breeze and customizable colorscheme contrasts.

See teams/vdg/issues#108 (closed)

Edited by Akseli Lahtinen

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