Update Dashicons.

New icons: `.dashicons-external`, `.dashicons-editor-contract` and `.dashicons-universal-access-alt`.
Updated icons: `.dashicons-code`, `.dashicons-universal-access`, `.dashicons-arrow-x-alt` and `.dashicons-arrow-x-alt2`.
Restores also `.dashicons-post-trash` as an alias for `.dashicons-trash`, which is the new one.

props melchoyce, empireoflight.
see #26936.
Built from https://develop.svn.wordpress.org/trunk@27989


git-svn-id: http://core.svn.wordpress.org/trunk@27819 1a063a9b-81f0-0310-95a4-ce76da25c4cd