Rewrite the painting part of agandaitem

Summary:
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.

Reviewers: KDE PIM, KDE PIM: Junior Jobs, mlaurent, dvratil

Reviewed By: KDE PIM, dvratil

Subscribers: knauss, lbeltrame, kde-pim

Tags: KDE PIM, KDE PIM: Junior Jobs

Maniphest Tasks: T9420

Differential Revision: https://phabricator.kde.org/D15916