Use strings more conservatively

Use strings more conservatively

Also adds the ability to efficiently estimate the length of, and
write out to, UTF8 from QHashedStringRef.

Change-Id: I8b6226ba41d855246ddf7d6268f8045c92ae219e
Reviewed-on: http://codereview.qt.nokia.com/3764
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>