Cell.setCellChildren misbehaves on cells with direct text children

[MichaelDarr]

Calling Cell.setCellChildren on a cell whose direct children are text nodes causes the entire cell to be deleted.

I thought this would be solved by wrapping the whole setter operation (removal of all child nodes followed by insertion of new nodes) in an Editor.withoutNormalizing block. However, this wrapper did not affect the issue at all: omission does not break currently working cases and inclusion does not fix this bug.

I believe that this has to do with Slate's normalization layers, of which I do not yet have a complete understanding.