LP#1832897: improvements to eg-fm-record-editor

LP#1832897: improvements to eg-fm-record-editor

* Add a new optional attribute, preSave, for passing a
  callback to modify modify records (e.g.,
  to provide default values) before they are saved.
* Ensure that the components current copy of a record is
  discarded when the user closes or dismisses the modal

  This addresses an issue where editing multiple instances
  of records on an admin page could display (and/or flash)
  stale data, particularly for the new multi-select widget.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>