Implement support for post-edit with VisualEditor.

This preserves the existing API by introducing a private post-edit
variable that is set from the 'postEdit' hook.  That hook is used
for listening to both wikitext and VisualEditor.

Change-Id: Ib0e82866ed7306cbd8491a9719bbcfc7f0299773
Merge "Implement support for post-edit with VisualEditor."