support for the shadow and effectiveBackgroundhints

Summary:
support the automatic shadow hint

When the hint is shadowedBackground, do an automatic shadow with a shader, and
change applet's color to complementary

Test Plan:
Tested the functionality on various applets. To test, the digital clock is enabling explicitly background configurability

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25591