Implemented Vertex Normal Calculation

Implemented the calculation methods for vertex normal calculation. They
can now be calculated after the data structure for the model has been
set up.
Renamed a few files

renamed a few files that could be misleading and to be more precise to
their meaning.
Implemented some fixes

Implemented some fixes that came with the integration in fusee. Most
fixes are related to prevent app crashing when corrupted models are
loaded. Most of the corrupted models will now be loaded with "holes"
instead of crashing the app.
Started to implement a little support for texture mapping

Implemented the uv part in the importer, texture can now be mapped even
if not correctly mapped.