import/export panel visibility with js

Summary:
Both when exporting and importing plasma layout to json
consider the panel visibility as well together the other
properties
BUG:374226

Test Plan: tried an exported layout with autohide panel

Reviewers: davidedmundson, Plasma

Reviewed By: davidedmundson, Plasma

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3852