Implemented Rendering components hierarchy.

From now on, every components that renders something into screen needs to inherit from components::Renderer. This will let us have a collection of components::Renderer to which we can make render using render(Vector2D& position) instead of having different collections for each type of rendering component.

This is a step toward implementing parallax scrolling.
Added Parallax testing state to test parallax feature.
Added Parallax Scrolling functionality.

The Parallax Scrolling is archieved by using a float from 0.0 (static background layer) to 1.0 (normal layer). This factor is multiplied by the camera correction vector when the position of the objects is calculated.
Merge branch 'rendering' of https://github.com/Poles/Poles into rendering

Conflicts:
	.gitignore
	CMakeLists.txt.user
Fixed the Parallax Scrolling displacement.
Added Parallax Scrolling feature to the text rendering.

Also the current FPS of the game can now be shown by pressing 'f'.
Added Parallax Scrolling feature to the text rendering.

Also the current FPS of the game can now be shown by pressing 'f'.
Fixed problem with SpriteSheet resources liberation.
Merge branch 'rendering' of https://github.com/Poles/Poles into rendering

Conflicts:
	src/core/components/SpriteRenderer.cpp
Multiple camera system.
Implemented rendering order.

Now the elements to render are added to a list where are ordered using its parallax index. A lower index indicates a that must be renderer first, and a higher index to render later, so objects who are nearer to the camera will be renderer later and will be over those with lower parallax index.
Changed rendering order from parallax index to z-index.

Now the rendering order is set by the z-index variable and the parallax index only controls how much the camera movement affects the position of an object.

This abstractions let us to make an static image (parallax index = 0) which can be renderer over all the other objects (high z-index, ie: 99). This can be used for UI elements, for example. It was imposible to archive this with the previous implementation.
Merge branch 'rendering'