Updates to valueWatch (relation label styling) and relation label parsing

Treebank documents can contain `"nil"` as relation label value - we blank this so that our `unusedTokenHighlighter` can count properly.

`valueWatch`, which is used to bring relation labels into the tree, received updates and now receives an optional attribute to define the visual representation of an falsy values. Such falsy values are also displayed in bold font-weight now.

Current implementation prints a bold `UNDEFINED` when a relation label isn't set.