correctly forward status from applets to containment

Summary:
unfortunately hiddenvisibility is the "biggest" status
that makes computing the proper status tricky.
if an applet in the containment is hidden, we must not mark as
hidden the whole containment, because it's as if hidden was
(as it should be) the smallest in the enum

BUG:372062

Test Plan: touchpad plasmoid no longer hides the systray

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4174