Identifiers (and filenames) need to be valid

[christofs]

It looks like at the moment, it can happen that the values of the "xml:id" are not correct, leading to XML documents that are not well-formed.

The source of the issue appears to be that the "xml:id" is generated from (among other things) the numeric IDs generated by Conftool. When this ID is placed at the beginning of the "xml:id", that value is incorrect, because it is not allowed to start with a number.

This issue might be related to issues #63 (URLs) and #57 (filenames).

One solution would be to always add the abbreviation of the conference as a prefix to the ID, e.g. "DH2022_123_Dupont.xml".