[dev-context] alternate descriptions

Hans, many thanks for having added the alternate descriptions functionality to ConTeXt. I attach the patch, since the entry reads /AltText instead of /Alt. At least, /Alt is the right value in PDF-1.7 (https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008....). I cannot check PDF-2.0, but it would be weird to break backward-compatibility here. Many thanks for your help, Pablo