global menu,rename menuHidden to visible

Summary:
--as @mart requested we can rename "menuHidden" property
to "visible" in order to follow Qt naming

Test Plan:
--check that global menu applet is functioning the same
way as when using removed menuHidden property

Reviewers: #plasma, mart, davidedmundson

Reviewed By: #plasma, mart

Subscribers: mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17107