add a new property in containment, for an edit mode

Summary:
used for opening the handles when the toolbox is open
or potentially other places

Test Plan: handles open

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D5153