We used a small buffer to preprocess a text to insert in text area (add
indentation, remove empty code points, compute the next cursor location),
but the size of this buffer was sometimes too small and caused a crash.
Now we do all the text odification in place in the text area buffer.
This makes the code in the various toolbox more generic. The arguments
of the text to insert are each replaced by a Ion::Charset::Empty.
These Empty chars are used to create layouts in ExpressionLayoutFields,
and to compute the position of the cursor in other fields, before being
removed.
Change-Id: Ie81c1e394b06fef2ab801ccff919d6550f70ec30
