[e] (0) Split off contentEditable DOM aspects for resuse elsewhere.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26329
Affected topics: DOM APIs, HTML

git-svn-id: https://svn.whatwg.org/webapps@8788 340c8d12-0b0e-0410-8428-c7bf67bfef74