Allowance for plugins to self-update

[Xjph]

Need to investigate the possibility of plugin-self updating. Plugin libraries can't be overwritten while Observatory is running, but should be able to allow them to at least check/download, and possibly queue up an update on next application start.

[pgrimstrup]

Suggested feature related to plugins:

Allow plugins to implement IDisposable and call the IDispsable.Dispose method when the application shuts down or the plugin is otherwise being unloaded. Some worker plugins may need to run background tasks and it would be nice to perform a clean shutdown rather than checking Environment.HasShutdownStarted.