New abiword exporter

Hi,

I tried to rewrite the abiword exporter using GsfXMLOut so that we don't
directly write xml code directly there. See attached diff file. Changes
for the .h file are trivial and the new IE_Exp_XML class does the dirty
job. This still needs testing.
Joining new and modified files (except Makefile.am
I probably should investigate to rewrite .odt and .docx export too so
that we can avoid bugs like #13707, but this is another story.

I also intend to test using GsfXMLIn for loading, by the way.

Cheers,
Jean