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

git-svn-id: https://develop.svn.wordpress.org/trunk@30338 602fd350-edb4-49c9-b593-d223f7449a82