Detect double-addition of an attribute to SimpleDTD.
Used scoped attributes.

In Alpino trees, there is only one relevant element: node. This allowed some
simplifying assumptions: we count just check whether an attribute was known
and if the corresponding value is valid.

However, TüBa-D/Z has multiple tree node elements (e.g. node, word, ne,
relation), which have overlapping attribute names, but not values. To
support such treebanks, the attribute check is now scoped by element name.
Add DTD for TüBa-D/Z.

The DTD was semi-automatically generated.
XPathValidator: parse DTD when switching corpus.
StatisticsWindow: re-read possible attributes when switching corpus.
Use TüBa-D/Z DTD when opening TüBa-D/Z.