General events

[matejpavlovic]

Start generalizing event representation in Mir, loosening the dependency on protobufs.
Define Event as an interface, make the core of Mir only use this interface type, and allow any packages to implement this interface. The protobuf implementation would only be one such package.

As a proof of concept, add a new version of the Ping-Pong toy application that uses custom Ping and Pong events / messages.

The biggest challenge is not as much the core of Mir, which is rather simple and uses events in a very general way, but some general-purpose modules. Those should, ideally, be usable by as many other modules as possible, without locking those other modules to the usage of any specific event types. This has already partly been addressed for the grpc networking module, but in general there is still a lot of work to do.