Add back arrow for image insertion
Move the blue arrow handling to semantic block module

The class to which the css is glued, is handled by the semantic block module,
and all semantic blocks that use the insertPlaceholder and
insertOverPlaceholder machinery will get it for free.
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.
Make blue arrow more translucent.
Remove a placeholder if any dialog is closed.

This meant hooking to the cancel buttons and the escape keyboard key.
Use $.on('click', instead of $.click. Deprecation and all that.
Merge pull request #145 from oerpub/recover-blue-arrow-for-images

Insertion arrow behavior update