[Folder View] Show "Widgets unlocked" help only for containment

When removing a widget it turns system-immutable, when undoing it regains its old immutability
triggering the notification.
Rather than tracking whether immutability changed from system- or user-immutability, just don't
spawn the notification if not a containment, which makes sense anyway.

BUG: 387589
FIXED-IN: 5.11.5

Differential Revision: https://phabricator.kde.org/D9222