GameContext for IoC and DI

[Animiral]

There are currently multiple ways to instantiate (one or several sets of) player objects, i.e. the objects that manage one player's stage (pit, cursor etc.) and logic.

• struct PlayerObjects in the screen module
• DemoFactory in visualdemo
• different *Test classes' SetUp functions

These methods overlap functionally with class StageBuilder.

In this issue, unify all creation methods behind one interface with minimal dependencies, probably called Repository.

It might feature the functions:

• reset, which initializes objects for the given number of players expected in the game
• get_*, yielding the appropriate object for the given player index

[Animiral]

There will not be an “object repository”, but the original aim of this issue will be served by the introduction of a class GameContext, like the GlobalContext recently introduced. It will hold the objects that are necessary to run a game round, like the GameState, Journal and Client.