Enable to set formatter and editor on the same cell

[gratex]

This enhancement makes possible to use formatter on editable cell too.
In previous implementation it was impossible, because when formatter was set on cell, renderCell method was always ignored.

In this enhancement we moved formatter resolving into our defaultRenderCell method.
This method is being called from Grid's renderRow and also from Editor's renderCell method, when Editor has property editOn set to "click" or "dblclick".

[kfranqueiro]

Thanks for this contribution - it's something that I'm kicking myself for not getting around to look at sooner. There was one subtle regression in behavior specifically involving formatters being (or not being) invoked on undefined values that I've tweaked the code a bit to remain consistent, and I've added cases to the editor and tree test pages to demonstrate the new functionality. I'll update the wiki shortly, since this was long documented as a limitation.