add backgroundHints for the panel view

Summary:
This patch adds a variable called "backgroundHints"
which can be used with (NoBackground) through qml
in order to enable/disable the KWin effects for
a specific panel and the panel shadows in case the
effects are disabled.
Currently these effects are (Blur and Background Contrast)

Reviewers: Plasma, #davidedmundson

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3282

FEATURE: 368384