Fix ApplicationScreenshots shadow for Breeze Dark.

Summary:
If you're using Breeze Dark or probably any other dark theme, the screenshot shadow-gradient will run from white to transparent. With this patch, the shadow runs from the currently set background color to transparent.

Before:

After:

Reviewers: apol, ngraham, Discover Software Store

Reviewed By: ngraham, Discover Software Store

Subscribers: davidedmundson, acrouthamel, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10701