Support XML files with newer document type versions (but old structure)

[gerking]

Latest versions of UPPAAL store XML files in version 1.6 of the corresponding document type definition. The grammar is currently restricted to version 1.1 and 1.2. This prevents newer files from being parsed, even though they use only language constructs that are themselves supported by the grammar.

The issue has already been reported for older versions of the project.

[gerking]

The current solution causes problems when serializing models. The serializer does not “understand” which DTD version number to select and write to the XML file. This is a little unforeseen as the previous version of the grammar also contained two alternative DTD versions (but these were written out in full tokens, repeating the surrounding XML structure). In this case, the serializer worked as intended by selecting the first of the two alternatives (which was version 1.1).