Define "new" engine modules and their interfaces

[TheJJ]

For the required architecture changes and in preparation for multiplayer games, we need to define needed components and how they talk to each other.

This is the main architecture-replanning step! The coarse goal architecture is stored in https://github.com/SFTtech/openage/blob/master/doc/architecture.md#information-flow.

Now, we need to design it in more detail. Of course, such a design is impossible to get at the first attempt, so we'll have to create it in a "changable" way.

[TheJJ]

with #1518 this is basically done :)