Add an unloadable property to GPluginPluginInfo and add a shutdown parameter to gplugin_unload.

Add an unloadable property to GPluginPluginInfo and add a shutdown parameter to `gplugin_unload`.

This allows plugins that can only be shutdown once to handle things properly. Examples of this are basically anything that pulls in another library that doesn't allow you to `_init` and `_uninit` more than once.

Testing Done:
Ran the unit tests and gplugin-query.

Reviewed at https://reviews.imfreedom.org/r/943/