Tweak how we populate NavBarItems.

Now we will always place child items of a node in the right bar, even if we're
also placing it on the 'top level nodes' list on the left.  This makes things
clearer for users by making it clear that any time you have a node selected on
the left, you can always find its children on the right.  And, it still preserves
the behavior we want where the left list acts as a high level 'table of contents'
for the file.
Ah, right, we pass in the source text, never mind. :smiley: