Offset are better to compute the length of a text range. The only problem
is that we get them for utf8 because that is the text representation of
source file. QTextDocument is Utf16 based, so it can not binary represent
the source file. Actually I do not see a simple performing workaround for
it.

Change-Id: Id615e1ee6a6e85c6ecc4f8044e275142409d9b46
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>