1. Now there's no fixed string length limit; see
http://www.autodesk.com/techpubs/autocad/acad2000/dxf/group_code_value_types_dxf_01.htm
The Autodesk's own DXF reference is inconsistent on this matter;
the "Group Code Value Types Reference" tells opposite to "About Group
Codes in DXF Files".
AutoCAD itself easily creates lines 10 000+ characters long!

This removes the hardcoded line length limit; also simplifies
internal struct DXFGroupReader used to parse DXF codes.

2. Improved text import: honor unicode strings in AutoCAD 2007+ DXF;
use more $DWGCODEPAGE values (note: when more codepages will be
discovered, they should be handled in DXFRepresentation::ReadHeader()).
Use LO locale when DXF doesn't specify one. Handle AutoCAD character
names.

3. General fixes & simplifications.

Change-Id: Ifde08978e2da7511696100520732bd76c1a660bb
Reviewed-on: https://gerrit.libreoffice.org/15627Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>