desk: Enhanced 8-bit spectrum bitmap

Created an attachment (id=7040)
The updated bitmap and the patch

Replaced the "spectrum_8.bmp" image with one that has better color dithering and also looks closer to the 16-bit color spectrum while at the same time visually telling the user that it is 256 colors. The height was changed from 4 px to 8 px in order to reduce the stretching of the bitmap.