possible to set expanded=false for a containment

Summary:
it will still have no visible effect, but
is possible to read and write the property if
containments make use of it (the systray uses it for
opening/closing its popup)

Test Plan: setting expanded : false to containments doesn't break them

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4866