Indicate which folders contain npjson/netpyne format files (and so can be opened) in dialog

Currently there's no way to see whether a folder contains *.npjson, and so you have to guess whether it contains a model. Simplest solution would be to change color of those folders, or add an icon in the title

Start by locating the dialog that displays folders and the existing handling for *.npjson files. Confirm how folders are inspected, then add a visible color or title icon for folders containing an npjson/netpyne format file; done means users can identify openable folders without guessing.

Written by the indexing model from the issue text.