make editMode a corona global property

Summary:
The plans are to switch on/off the whole plasma shell into edit mode.
For this it needs to be a global corona property, rather then just
containmentInterface.
Plasmashell would expose a dbus call to set it from systemsettings

Test Plan: plasmoid.editMode=true still works

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, GB_2, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24239