3d-models in .obj format

[VilleK]

Added rudimentary support for loading .obj 3d-models.

Limitations:
- Only triangle surfaces supported
- Textures are not imported
- Diffuse color is the only material property supported

Also added support for drag-drop model files (.3ds and .obj) from Explorer to ZGameEditor tree to make them easier to import.

http://www.zgameeditor.org/files/ZGameEditor_beta.zip