This class had slightly confusing semantics and the added weirdness
doesn't seem worth it just so we can say "." instead of "->" when
iterating over a vector of NNRPs.
This patch replaces NonnullRefPtrVector<T> with Vector<NNRP<T>>.
Having to subclass GUI::Model for even the simplest type of hand-built
TreeView makes them a bit unpleasant to work with at the moment. :^)
This adds such a GUI::Model subclass that is specifically designed for
adding nodes to a TreeView manually, supporting text and an optional
icon by default, and allowing for further data when subclassing the Node
class.