update panel shadows when background hints are changed

Summary:
--when the containment background hints are updated then the
panel shadows should be also. In case the background hints
point to NoBackground the panel shadows should be removed
and in case they point to DefaultBackground they should be
drawn again
--PanelShadows::removeWindow has no place in updateEnabledBorders
because the window still lives. When removeWindow was executed
before it was faulty unregistered from PanelShadows and in order for
setEnabledBorders to work again we should need
to execute from start the PanelShadows::addWindow function

Test Plan:
--I have created an applet which a PlasmaComponents.Switch button
and toggles panel transparency on user demand easily

Reviewers: #plasma, davidedmundson, mart

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25121