Big new Feature/fix for previous feature
Now with Enter you can jump from one frame to the next in an image and it will open and select the same polygroup you were on (if possible) and set you on the first polygon if one exists
else it will just select the group.

When you're drawing boxes in the image view itself as well, if you are done and want to go to the next frame, just hit enter and you will jump to the next frame and if possible, be on the first polygon in that group
(there's no guarantee that there is the same amount of polygons in a group from frame to frame so I can't set you on the same frame and that seems a bit redundant anyways)