Section editor enhancements, refactoring

* Create a class for the UX of section editor CXSectionEditor
* Refactor the MediumEditor binding to an editor enhancement
* If the user clear the translation in a section, present the placeholder
* Define custom event mw.cx.translation.clear for the above mentioned case
* Refactor the placeholder construction code for the above purpose
* For Figures, make the figcaption alone editable, prevent return key press
  in figcaption.
* Avoid previews on mouse over of links

Change-Id: I960c8530055a3fc69cdcf3a30090a1179e4a5709