Changes to keep better track of placeholder.

* insertPlaceholder now returns the jquery object of the inserted placeholder.

* no more magic cleanup of old placeholders that may be left behind, plugins
  should clean after themselves.

* insertOverPlaceholder now requires that the placeholder be passed to it.
  Explicit is better than implicit.

* Changes to figure, image and media plugins to accomodate the above.
If user cancels, remove the placeholder.