Started implementing Model's LoadFromFile

|SOURCE|
[Common/DataTypes]
* Added a FILE_CHUNK struct for files to use, each chunk has an ID and the
  number of bytes which follow it
* FILE_END_CHUNK should be present at the end of each chunk, it's a crude kind
  of error checking
[Common/Model]
* LoadFromFile will load a hypothetical model's header, then crash as there is
  no code to read data from chunks
* Added a MODEL_HEADER struct containing vital information for creating a model
* Added a MESH_DATA struct containing the ID of the mesh and the MD5 digest of
  the material to apply