Copy Editor.{cpp,h} to SpellChecker.{cpp,h}

This patch copies Source/core/editing/Editor.{cpp,h} into Source/core/editing/SpellChecker.{cpp,h} for decoupling Editor and spell checker.

Another patch will complete decoupling.

BUG=n/a
TEST=n/a; No behavior changes.
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/26795002

git-svn-id: svn://svn.chromium.org/blink/trunk@159304 bbb929c8-8fbe-4397-9dbb-9b2b20218538