[Containment Interface] Don't enter edit mode when immutable

Opening the toolbox when widgets are locked reveals the widget handles but they're empty as all the actions are unavailable.
Just refuse to enter edit mode then.

Differential Revision: https://phabricator.kde.org/D12379

src/scriptengines/qml/plasmoid/containmentinterface.cpp

@@ -422,6 +422,10 @@ void ContainmentInterface::setEditMode(bool edit)
         return;
     }
 
+    if (m_containment->immutability() != Plasma::Types::Mutable) {
+        return;
+    }
+
     m_editMode = edit;
     emit editModeChanged();
 }