inspector: Do not show transient nodes

inspector: Do not show transient nodes

This is kind of a hack the way it's implemented, but it's necessary
for performance to ignore transient nodes as they get created all the
time (via gtk_style_context_save()) and invalidate the whole treeview.
And that causes resizes and redrawing of the treeview and performance of
the inspector would go down the drain now that we display a larger part
of the node tree.