Allow setting separate corner radii for ShadowedRectangle

Summary:
This adds a "corners" grouped property that allows setting separate radii for
the different corners of ShadowedRectangle. By default, these are ignored and
instead the value of "radius" is used.

Test Plan:
The manual test has been updated, it should look like this:

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: broulik, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28314