Fixing some inconsistent behavior on toolbar icons:

- Use .ab-icon:before, .ab-item:before, and #adminbarsearch:before to select all toolbar icons.
- Rewrite the selector for toolbar icon hover styles so they're not overridden by default colors.
- Add #adminbarsearch:before to color schemes, so its icon color is changed with the others.
- Remove duplicate color rule on #adminbarsearch:before.

See #25858.



git-svn-id: https://develop.svn.wordpress.org/trunk@26184 602fd350-edb4-49c9-b593-d223f7449a82