DevTools: rename SourceEditor to SourceView, make entire SourcesPanel a drop target.

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/211003002

git-svn-id: svn://svn.chromium.org/blink/trunk@169938 bbb929c8-8fbe-4397-9dbb-9b2b20218538