Increase resolution of Windows icon.

The .ico file has changed in the following ways:
* Added 64x64 layer (max size for "Classic Mode").
* Added 256x256 layer (max size for Vista and 7).
* Removed copies with no alpha channel:
  * Display depths lower than 32-bits are rare nowadays.
  * 8-bit alpha channels in icons has been supported since XP.
  * If the display depth is lowered, they look no better than the
    downsampled versions that Windows automatically generates.

Tested various sizes on both XP and Win 7. It looks fine
(unchanged) on XP and downright sexy on Win 7.