DevTools: Extract SourcesEditor from SourcesPanel.

This extracts everything related to our tabbed editor from the sources panel to the separate SourcesEditor class.
This class could be later refactored into the view that could be embedded in other places (e.g. Drawer).

R=pfeldman, lushnikov

Review URL: https://codereview.chromium.org/206063003

git-svn-id: svn://svn.chromium.org/blink/trunk@169653 bbb929c8-8fbe-4397-9dbb-9b2b20218538