Don't allow to add/remove monitor-section w/o engine

Don't allow to add/remove monitor-section w/o engine

Adding/removing the Monitor section changes port-connections,
a port-engine is needed to establish connections.

Ideally we'd allow this and queue for connection changes. This
will however require deep changes.

Also toggling the monitor-section on/off/on w/o engine can later
result in failure to register ports. Due to lack of engine-signals
not all shared_ptr<> port references are dropped.