Fix bug #78: bboxes won't save

[e-platini]

See https://github.com/smistad/annotationweb/issues/78

EDIT: Not a bug. It was normal behavior to prevent accidentally adding keyframes by clicking on the image when treating sequences. However, in the case of annotating single images, it is quicker for the user if they do not have to add a keyframe for every image.

If the sequence length is 0 (meaning there is only 1 image), then adding the current frame number as key frame when adding a box.