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
{F7788551}

Reviewers: #plasma

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

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