Shutdown plugins when terminating the server

[Enet4]

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Although we have a Dicoogle plugin set interface exposing a shutdown() event method and a shutdown() method in our plugin controller, none of these are actually being called. Maybe it was, back in the remote GUI version, but not anymore. We should probably hook this to a termination signal.

[bastiao]

@Enet4 seems that we going to address this at #115. So it may make sense to close this one?

[Enet4]

#115 does not fix this bug. It proposes a different way of implementing plugins, but that alone does not ensure that a shutdown always takes place. In addition, I am still hoping that this can be done without a major version of Dicoogle.

[Enet4]

Dicoogle has behaved like this for so long that it's best to push this to the next major version, so as to prevent surprises.