Replace all Wavefront occurences with 3DModel

[Xpirix]

For now, the 3D Model resources are referred to as Wavefront in the code. However, according to qgis/QGIS-Django#388 (comment) and qgis/QGIS-Django#202 (comment), it's not self-explanatory and they should be referred as 3DModel to be more clear.