Suggested gui improvements.

[cudmore]

Can we get the following:

1. mouse wheel will scroll up/down through images/slices in a stack.

2. option/command + mouse wheel will zoom in/out.

This is much better for the user experience. Especially when making new spines and seeds for tracing segments.

Once the user chooses a zoom, they can rapidly scroll up/down through images to choose the image plane/slice to create an annotation in.

The vertical slider to select/view image planes should remain. It is just better to have the mouse wheel go up/down through image slices.

Finally, can we get a comment in the code where this change in gui is implemented? Post the text of the comment back to this github issue when done.

Otherwise, looking awesome!