EZP-20069: Expose SignalSlot repository properly

This improvement exposes slots in SignalDispatcher using a `ezpublish.api.slot` tag.
This makes SignalDispatcher extensible, since one can now register his own service as a regular slot for a given signal.

### TODO
* [x] Make it possible for the slots to register to SignalDispatcher via a service tag
* [x] Move legacy related slots definitions to LegacyBundle
* [ ] Implement a generic slot that convert slots to regular Symfony events.