Changes towards enabling search to be extended by Media bundle

This PR:

- allows SuluSearch metadata objects to be modified by event listeners.
- prepends a configuration to MassiveSearchBundle to use a custom Factory object so that we can create our own (Sulu) classes.

This is required to enable bundles such as MediaBUndle to remain uncoupled from SearchBundle.
I think this made the most sense. So we dispatch a specific event when `Structure` metadata is loaded, this will enable the `MediaBundle`, for example, to identify which properties in a document are image fields.