This is needed to update the GUI (mixer-strip I/O buttons)
when the engine is re-started with different I/O.
When Port::reconnect() cannot re-connect the port (e.g.
because the corresponding hardware port is missing), the IO object
needs o be notified -> PostDisconnect() -> IO::disconnect_check.
While the port-manager re-establishes and re-connects port,
port_manager->running() == false, and Port::get_connections()
returns the Port::_connections set (not actual engine connections).
So _connections.clear() has to be called synchronize it with
the actual engine connection state.
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