Entities

[Unarelith]

The goal of this PR is to add entities to OpenMiner.

Checklist:

• Use entt and add a Scene
• Add some controllers
• Use a factory to create an entity (item drop for example)
• Server-side entities
• Find a way to have a safe server-side entity counter (maybe use server-side entity id instead?)
• Remove useless files in cmake/
• .travis.yml shouldn't compile and install SFML and GameKit before OpenMiner anymore
• Create subfolders for components and controllers
• Client should reload its registry when changing dimension

[Unarelith]

Most of the work has been done and the branch is stable now. I'm merging this.

[FreeLikeGNU]

ooo cool!