Registrables
The RegistrableProvider is a holder for all registrable objects that your plugin provides to the API. The usage for this class is the reloading process - it would be difficult to reliably unload and load back registrables by a certain plugin without a constant provider holding everything at all time.
The usage of this interface is very simple: it can be implemented anonymously and then fed to the registry at any time.
The implementation of the registry is such that upon providing a registrable provider, the registry ensures that the associated plugin is enabled and then immediately registers all the provided registrables. Upon reloading the plugin, all registrables are dropped; the registry then goes through all providers and loads the provided registrables if and only if the plugin is enabled. If not, the provided registrables are skipped.