Entity representation rework

[armandfaux]

Entities were first designed to be a class in the engine, because classes are easier to visualise and understand.
Each entity currently owns its own components. While this is a simple and easy to use method, it's not optimised and causes much overhead.
I want Kaelinn to be faster and a rework of the entity management seems mandatory at this stage.

• An Entity will no longer be a class, but rather a mere "id" (unsigned int)
• This id will also be an index for accessing the entity's components and its signature

This is likely the biggest change ever made that deep in the engine structure. However, significant performance improvements are to be expected.
This rework will cause many side effects, since every module will be affected. The most critical parts of the code that need refactorisation are the following :

• How and where are the components stored (no more entity objects to store them)
• How system interact with entities and components
• How we create, delete, and modify entities
• How we keep a track of the entity "type"

See #19, #20