Introduce Distraction-Free Writing v2, a re-think of DFW that uses the main editor instance

* the "DFW" button is now an auto/off toggle
* defaulting to auto during beta, decide later for release
* "auto" means that DFW gets enabled as you start typing in editor
* tabbing and mousing out will bring the full interface back
* there is a slight grace period during which your mouse can quickly return

Feature plugin work happened here: https://github.com/avryl/focus

props avryl, azaozz, Michael Arestad
fixes #29806
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git-svn-id: http://core.svn.wordpress.org/trunk@30337 1a063a9b-81f0-0310-95a4-ce76da25c4cd