Redesign the "grey" color scheme:

- Remove rounded borders.
- Fix padding problems due to the new access checkboxes.
- Change dialog headers to grey.
- Change the "close dialog" button to the grey X icon from Dashicons and change the colours to match WordPress standard.
 - Rename "gray" to "grey".
 - Fix a couple of other layout issues.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@948699 b8457f37-d9ea-0310-8a92-e5e31aec5664