Previously Port::PortDrop was never handled.
The signal was disconnected directly when the connection
is re-used by Port::PortSignalDrop.
Ports::drop() was not called when the engine was stopped
or disconnected, and port-handles were not invalidated.
This lead to crashes whenever a port-related operation was performed
while the engine was stopped. e.g. adding/removing tracks or plugins
(latency recompute, notify port-engine) and various other operations.
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