Rewrite the painting part of agandaitem
Remove gradient in favor of a solid color
Rewrite how the corner radius and border are painted.
Test Plan: Compile and run in the 18.08.1stable branch. Still need testing in master.
Reviewed By: KDE PIM, dvratil
Subscribers: knauss, lbeltrame, kde-pim
Maniphest Tasks: T9420
Differential Revision: https://phabricator.kde.org/D15916