Design and finish engine's API

[Witek902]

Implement missing parts of the engine's C++ API.

The API should:

• be very well documented (in doxygen style)
• provide all functionalities the engine offers:
  • creating and managing ALL entities types (and whole scenes)
  • creating and managing ALL resources types (each resource should be "editable" from the application's level, for example to generate custom meshes and textures at the runtime)
  • handle miscellaneous events (scene events, physics, resource manager)
  • change engine's configuration (renderer as well)
  • calling (de)serialization functions
• be idiot-proof (handle all errors and log as much as it is possible)