RFC: Accent colour for KColorScheme

Summary: This is a roughly thrown-together patch demonstrating how an accent colour can be implemented via modifiying KColorScheme.

Stuff I plan to do:

• Figure out a way to more elegantly handle inactive and disabled accent colours

Test Plan: Add Foreground and Background colors to [Accent] in kdeglobals

Reviewers: #plasma

Subscribers: The-Feren-OS-Dev, ngraham, kde-frameworks-devel, ndavis

Tags: #frameworks

Maniphest Tasks: T11052

Differential Revision: https://phabricator.kde.org/D27263