[KStyle] Set the color of KMessageWidgets to the correct one from the current color scheme

Summary:
KMessageWidget has to rely on hardcoded colors because it's in a Tier 1 framework.
However after determining its color it will give the style a chance to set a
better fitting color by calling polish. So set it's color to the appropriate
one from the current color scheme.

Test Plan:
KmessageWidget follows Color Scheme

Reviewers: Frameworks, aacid, anthonyfieroni, apol

Reviewed By: anthonyfieroni, apol

Subscribers: anthonyfieroni, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27039