By implementing this trait anyone can provide his own drawer, for instance one that draws onto
a bitmap, if he don’t want to use the SvgDrawer used by the crate by default.
The Visualize trait abstracts the visual presentation of the node’s data.
It can be implemented by the Tree<T, …>’s node type T when custom visualization is desired.
Only mandatory to implement is the visualize method.
The Embedding is the interface to drawers that need the embedding for the purpose
to transform it to their own output format.
It is accessible at the Layouter after calling an embed method