Default att values incorrectly processed by Roma web app

[peterstadler]

relocating from TEIC/TEI#1213

---

When an attribute has a specific default value in the ODD source, and the user attempts to change the value list for it using web roma, the form does not display the existing default value. Consequently, the user has no way of knowing that it exists, and may easily supply a list of attribute values which do not include it, thus generating an invalid DTD or XSD schema. Tested this with which retains the default "simple" even though it does not appear on the screen.

Original comment by: @lb42