Add opt-in `call-shutdown` setting to call plugin shutdown routines on Dicoogle shutdown

[Enet4]

Although I was originally thinking about bringing the shutdown hook to call shutdown() on all plugin sets in Dicoogle 4, we felt that there was a need to have this earlier. This PR makes this capability opt-in, so that it does not require any breaking changes and does not lead to surprising behaviors.

Resolves #149.

• New server settings XML element <call-shutdown> in <archive>
  • if true, Dicoogle will call shutdown on (not dead) plugin sets installed
  • this is false by default, but the plan is to make it true by default (opt-out) in Dicoogle 4, and maybe even deprecate it, so that in later versions the behavior becomes mandatory.
  • add test coverage for call-shutdown setting
• add shutdown hook to call PluginController.shutdown
  • Although we've had issues with this when interoperating with Sentry, we currently believe that this functionality is more important than Sentry integration.
• improve PluginController.shutdown() by catching and reporting all exceptions individually