WORLD WAR Z-INDEX: Restoration of sanity to revisions/slider/menu z-index values.

The year is 2013. It is a period of DOM unrest. Sliders have hoarded
all available z-index, leading to an arms race. What started as a local
squabble soon escalated into a global conflict. The flyout menus were
the first of the collateral damage, trodden underfoot by mighty warring
enemies they couldn't effectively challenge. It was said they were
betrayed by one of their own: the z-index-power-hungry non-active
currently-hovered submenu, whose z-index-power level was over 9000!

But there is hope. A small band of heroes has emerged. Using their
powers of "what were you thinking?" and "chill, no one needs a z-index
THAT high", they have begun to restore order and harmony to the DOM.

See #24736. Props markjaquith, aaroncampbell, helen.

git-svn-id: https://core.svn.wordpress.org/trunk@24753 1a063a9b-81f0-0310-95a4-ce76da25c4cd