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.


Built from https://develop.svn.wordpress.org/trunk@26184


git-svn-id: http://core.svn.wordpress.org/trunk@26093 1a063a9b-81f0-0310-95a4-ce76da25c4cd
We only need to declare Open Sans once in wp-admin.css. See #25858.

Built from https://develop.svn.wordpress.org/trunk@26185


git-svn-id: http://core.svn.wordpress.org/trunk@26094 1a063a9b-81f0-0310-95a4-ce76da25c4cd