- ResourcesManager class is now a template class expecting the type of held resources as an argument.
(*) NOTE: The client program doesn't compile on this commit!.

- Added LocalLightsSelection and LocalMeshesSelection classes.
- Added an empty constructor for DrawablesManager class.
- DrawablesSelection and LocalDrawablesSelection now inherit from ResourcesSelection<Drawable> and LocalDrawablesSelection<Drawable> respectively.
- LightsSelection and MeshesSelection now inherit from ResourcesSelection< LightProperties > and ResourcesSelection< Mesh > respectively.
- ResourcesManager template class now requires 3 template arguments.
- ResourcesManager template class now inherits from Observer and Observable classes.
- Now is responsability of ResourcesManager to keep a map of ResourcesSelection<T> instances, as well as a pointer to a LocalResourcesSelection<T> and a pointer to a ResourcesSelection<T> with non-selected resources.
- Removed duplicated code.