MTEXT: make strings translatable.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
MTEXT: check for NULL pointers and do basic checks.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
MTEXT: changed the maximum number of parameters.

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

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