edit mode action and dbus

Summary:
edit mode menu action
edit mode dbus property for systemsettings
containmentlayoutmanager goes out of edit mode only when the whole
plasma app loses focus, so the panel controlled doesn't make it go out
of it

Test Plan:
iaction shows in context menu, works
property exposed on dbus works

Reviewers: Plasma, VDG, ngraham, GB_2, davidedmundson

Reviewed By: Plasma, VDG, ngraham, GB_2, davidedmundson

Subscribers: GB_2, ngraham, davidedmundson, plasma-devel

Tags: Plasma

Maniphest Tasks: T10190, T11094, T10402

Differential Revision: https://phabricator.kde.org/D24264