Create Windows icon from image

This is an updated version of the already existing iconify2.scm.
Iconify2 plug-in converts a single layer of a single image into a multi-layered image ready to be saved as a Windows icon.

Modifications:
- Tested for Gimp 2.8
- Also creates the 256x256x32 big icon for Windows XP and later
- Slightly better naming of the layers
- Cleanup of the temporary images