Fix cancel editing logic

The old version of this code referred to a variable that did not exist
(cellEditorIsModalWindow) and resulted in a dead code path. In addition,
if that path were ever taken, it attempted to change an attribute of a
  nulled variable.  This has been rewritten to match what the original
  author's intent probably was.
Make cellEditor protected (was private)

I made a pretty strong case for making cellEditor a protected variable
here: https://github.com/qooxdoo/qooxdoo/pull/87

The gist of it is that in order for the pane scroller to be extensible,
it is pretty important that the cellEditor variable be visible to
descendant classes.
Merge pull request #89 from AaronOpfer/protected_cellEditor

[BUG #8398] Make cellEditor a protected variable to facilitate OOP