ENYO-1020: implement Phobos#saveAs

- UI interaction is handled by Phobos (text editor), which raises events
- Events are trapped at the Ares level, which implements async file-
  system operations & report their success back to Phobos
- FIXME: Phobos#saveAs() uses a common-JS type of completion callback
  (which is good) while Phobos#save() makes Ares directly call into
  Phobos's API (which is ugly, because Ares needs to know about an async
  operation completion public API which does not need to exist).
- FIXME: saveAs popup does not show the folder where the current file
  is.  This is Ok in case of multiple saveAs (operation resumes in the
  same folder as before), but not for the first popup.

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>