[WIP] Fix COCO import when the segmentation element is empty.

[wilderrodrigues]

When importing a COCO dataset that contains only bounding boxes, used for Object Detection, the application doesn't work.

This PR fixes issue #452.

After the changes applied, I was able to successfully import my dataset. See images below:

I'm not really into frontend and the JavaScript frameworks used in the tool. So, I need some help with the UI. Right now we have some issues:

1. We don't see the bounding boxes on the preview panel;
2. When we click on an image, the bounding box is not present;
3. The categories panels show that we have 1 category, but it's empty;
4. Going back to the preview panel will show that the image is no longer annotated.

Despite the issues above, the Export COCO works fine and keeps the bbox element properly field.

I hope someone help with the UI stuff. I think the project is interesting, but it should cover bounding boxes in a better way.

@jsbroks