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

[pgleeson]

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

[pgleeson]

Still an issue... This is implemented better in the NeuroML select dialog, where the SELECT button is disabled until you click on a file ending in net.nml. Similar could easily be done for folders containing a *.npjson file.