Node transformers do not visit inline nodes in definition-list terms #1425

The compiler builds its shadow tree from getChildren() only.
DefinitionListItemNode keeps its term and classifiers in properties of
their own, outside its children. A NodeTransformer therefore never
sees an inline node inside a term or a classifier.

A theme that completes inline nodes in a transformer gets different
output for the same role in a term and in running text:

:custom-role:`value`
    The definition.

A paragraph with :custom-role:`value`.

The role in the paragraph is transformed. The role in the term is
rendered exactly as the parser created it.

We worked around this in the TYPO3 theme: our transformer also supports
DefinitionListItemNode and walks the term and classifiers itself. Each
transformer that works on inline nodes has to do the same.

Possible fix: let TreeNode include the term and classifiers of a
DefinitionListItemNode when it builds the shadow tree, or make them
children of the item.