Basic support for TüBa-D/Z.
TüBa tree conversion stylesheet.
TüBa: Tokens are in 'word' elements.
Stylesheet fix (node is alway interior).
Mention TüBa.
Thinko.
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.
Add basic support for secondary edges.
Simplify path drawing.

With quadTo(), we only need one control point.
Secondary edge labels.
Make (primary) edges separate graphics items.

Primary edges were part of tree nodes. However, this had the disadvantage
that the z-levels of nodes and edges could not differ. Now that they are
separate items, the z-levels can differ, which we exploit to improve
displaying of secondary edge labels.

The ordering is now (from high to low priority):

- Nodes
- Secondary edges
- Primary edges

This has the result that a secondary edge label can be covered by a node in
rare cases. Some more sophisticated collision detection is necessary to
avoid this.
Fix z-value for popup items.
Add collision detection for nodes and secedge labels.

This tries to place secondary edge labels on a different position on the
edge if the preferred position (the middle) would result in overlap with
a node.
Cleanup: move const position.
TüBa-D/Z explode script.