[FIX] issue when tabbing too much at end of editable list row

Tabbing is intercepted by keydown_TAB, which — if the current cell is
the last active field of the row — will then call _next:476. _next
then calls save_edition:300 which "takes a lock" (more precisely
serializes access to its body) and within its body checks if an
edition is active (:303) and returns immediately if not (:304).

The problem here is when a second tab event arrives during the
potentially extremely long save_edition body (since for toplevel lists
it needs to perform a complete RPC call): the overall state of the
list has not changed so the second event *also* goes into _next, then
into save_edition. There it's serialized with the ongoing call and
thus inactive until said ongoing call's termination, and reaches the
body after the current edition has been wound down. As a result, the
body of _next (:408) gets the resolution of ``$.when()``, which is
``null`` and the first condition blows up.

There are 3 possible ways to fix this:

* adding a check in keydown_TAB's handler to see whether a _next call
  is ongoing. This requires adding a state flag to the object and does
  not protect (or cooperate with) _next calls from outside this
  specific handler, unless they are modified in turn.

* alter save_edition to *fail* in case there's no ongoing edition:
  this part was originally in ensure_saved which does not care whether
  a save was necessary or not and does not propagate save information,
  so ``$.when()`` made sense. In save_edition, there are really 3
  different outcomes: the save succeeded, the save failed (or
  potentially part of save's postprocessing failed, for the current
  implementation) and the save was unnecessary. But deferred only
  provide 1 bit of state (success or failure), so the last state has
  to be merged into either success or failure.
 
  Both make sense, to an extent. Changing from one to the other (as
  necessary here) could break existing code and have more extensive
  effects than expected.

* the simplest and least far-raging change is to just alter the
  save_edition().then handler to ignore cases where save_edition()
  results in no saveinfo, this can be assumed to be a
  bailed-out/unnecessary save call.

For simplicity, the 3rd solution was picked here although with more
extensive tests &al I'd have preferred trying out 2nd.