Open Image Files: allows duplicate section names when only file extensions differ

[sorghumking]

Image files with the same name but differing extensions are all added to the track list. For instance, when GLAD-1D-3H-1.jpg and GLAD-1D-3H-1.bmp are selected to import, both are added as GLAD-1D-3H-1:

Such duplicates should be resolved to a single section image before addition to the track list.