Add a darker search icon to the spritesheet.
This adds a darker version of the search icon, intended for lighter

backgrounds. It's otherwise of the same style.
For consistency, I placed this as the next icon after the lighter search

icon, shifting over some other icons.
I also shifted the light icon just a bit to better pixel-align it. This is

most noticeable on non-Retina resolutions.

Tested each affected icon in Retina and non-Retina modes.
Made use of the icon in another change.