[KStyle] Set the color of KMessageWidgets to the correct one from the current color scheme
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.
KmessageWidget follows Color Scheme
Reviewers: Frameworks, aacid, anthonyfieroni, apol
Reviewed By: anthonyfieroni, apol
Subscribers: anthonyfieroni, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D27039