Fix various issues when the engine disconnects:

Fix various issues when the engine disconnects:

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.