multiplayer: language blocks for "player joined" and "player left" events

[eanders-ms]

Online multiplayer will support join in progress. It would be nice if the game could be aware of the comings and goings of players.

The blocks would need to work for local multiplayer too. For example:

• "player joined (slot 2)" would fire the first time the second player controls are used in a game, or when input is detected on a second game controller.