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
{F7788553}

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

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