Feature request: Flicker between 2 images

[RaananHadar]

Lets assume that I have 2 medical images that are co-registered. For example:

1. A CT image.
2. The result of a segmentation algorithm on this image.

It would be really beneficial if it was possible to easilly toggle between the images by using a predefined keyboard shortcut or a mouse click. Now as these are medical images, each image has its own contrast stretch and its important not to override the contrast stretch using the settings from the other image.

[timholy]

One way you can do that now is to open two windows and link their axes together. Then hitting Alt-Tab should toggle between the windows. https://github.com/JuliaImages/ImageView.jl#coupling-two-or-more-images-together

[mgkuhn]

The single-stepping keyboard shortcuts suggested in #177 could probably also be used for that.