Cleanup of image loaders

- ImageLoaderWx now handles JPEG, BMP and PNG. The home-grown JPEG, PCX and PNG
  loading code is gone.
- TGALoader remains to handle TGA images since the wxImage loading code is
  broken and cannot load RLE-compressed TGA files.
- Removed the nasty module name hack to distinguish image loading modules.
  There is now only one image loading module, ImageLoader, whose interface
  exposes the two loader methods that were previously in the ImageFileLoader
  helper class. This module's implementation contains a number of
  ImageTypeLoader sub-modules internally, which are not exposed to calling
  code, with each module being able to handle any number of image extensions.