0030549: Coding - split Image_AlienPixMap into several classes

0030549: Coding - split Image_AlienPixMap into several classes

Image_AlienPixMap has been split into:
- Image_AlienPixMapI defining a general interface for reading/writing images from/to external files.
- Image_FreeImage implementing interface using FreeImage library.
- Image_WinCodec implementing interface using WinCodec library.
- Image_AlienPixMap redirecting to either Image_FreeImage or Image_WinCodec.

New static method Image_AlienPixMap::SetDefaultFactory() allows configuring
an external image library implementing Image_AlienPixMapI interface
to be used by default OCCT image processing routines (texturing, image dumps, etc.).