STYLE: added dxf_style_is_xreferenced().

Signed-off-by: bert <bert.timmerman@xs4all.nl>
STYLE: white space corrections in Doxygen comments.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
STYLE: added dxf_style_is_xresolved().

Signed-off-by: bert <bert.timmerman@xs4all.nl>