Unfortunately we store the state of models as simply model, so if there's ever
duplicate model names, we're somewhat screwed, but this makes the (previously
unmanageably huge) menu usable, while retaining the "model name as global
identifier" state unmodified.
Remove Canvas::Layout, use Canvas::Container for the same purpose, move child-rendering into Item::render_children() so that it
could theoretically be used by any derived type.
