handle action for custom shadow background hint

Summary:
The handle now uses ToolButtons to support the checkable background toggle
The new handle action enables and disables the background and switches it
to a shadow with the new effectivebackgroundhints

Test Plan:
Tested with different applets

Reviewers: Plasma

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25592