GCWU: Change menu images to gradients

Left the arrow icon for now, but will look at replacing it with a CSS
triangle
GCWU: Change search button image to gradients

Add cross browser gradient for button on regular and hover.
Also removed a commented out reference to the image that was left over
from the Base theme cleanup
GCWU: Change splash page buttons to gradients

Also removed sp-pe-images sheet since it was no longer used
GCWU: Remove unused icons from Intranet theme
Merge pull request #1726 from nschonni/gcwu-replace-gradient-images

CGWU Themes: Replace images with CSS gradients and oldIE fallbacks