use new property plasmoid.editMode

Summary:
toolboxes set editmode by themselves
possible for toerh things to trigger edit mode now

Test Plan: handles open correctly

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5154