Themes screen: Various IE8 adjustments and fixes.

Merges [27007] to the 3.8 branch.

props morganestes, matveb.
fixes #26541.

Built from https://develop.svn.wordpress.org/branches/3.8@27008


git-svn-id: http://core.svn.wordpress.org/branches/3.8@26885 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Responsive improvements to submenus in the toolbar.

Merges [27006] to the 3.8 branch.

 * Remove .ab-sub-wrapper rules in the 782px media query, as they're duplicates of the rules set on line 161.
 * Reposition the arrows for submenus at responsive sizes where the menus are taller.
 * Set the submenu position to static at smartphone sizes, so it's not positioned outside of its parent (and thus offscreen).

props undergroundnetwork, iammattthomas.
fixes #26720.

Built from https://develop.svn.wordpress.org/branches/3.8@27009


git-svn-id: http://core.svn.wordpress.org/branches/3.8@26886 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve keyboard accessibility for the admin menu when in responsive mode, props SergeyBiryukov, fixes #26639 for 3.8.1.
Built from https://develop.svn.wordpress.org/branches/3.8@27010


git-svn-id: http://core.svn.wordpress.org/branches/3.8@26887 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Themes screen: Add aria-describedby attributes to provide better information to screen readers.

Merges [27012] to the 3.8 branch.

props joedolson for initial patch.
fixes #26602.

Built from https://develop.svn.wordpress.org/branches/3.8@27013


git-svn-id: http://core.svn.wordpress.org/branches/3.8@26890 1a063a9b-81f0-0310-95a4-ce76da25c4cd
3.8.1-RC1

Built from https://develop.svn.wordpress.org/branches/3.8@27014


git-svn-id: http://core.svn.wordpress.org/branches/3.8@26891 1a063a9b-81f0-0310-95a4-ce76da25c4cd