Allow easier integration of phoenix in other applications

Note : I believe this is a way to make #824 possible without implementing it inside phoenix

Phoenix is a great tool to visualize and "inspect" physics event. On that base, one could imagine to build more targeted application, like master classes or analysis applications which would benefit of phoenix visualization and add more features. To give a practical example, the LHCb master class (https://lhcb-outreach.web.cern.ch/lhcbinternationalmasterclasses/d0-lifetime/) requires students to select particles in events which are candidates to a D0 decay and tries then to compute the mass of the D0 particle.

In order to use phoenix in such a context, one would need to define an interface to phoenix where the external application could easily register callbacks on the different phoenix actions (select in particular) and access associated data.

Start by reading issue #824 and tracing how Phoenix currently handles actions, especially particle selection, and exposes associated event data. Define the external integration boundary around callback registration and data access; done means a documented interface supports applications such as the LHCb master class without implementing the feature inside Phoenix.