Added button-based navigation of images.
Fixed behavior of `showPrevious` and `showNext` methods.

The two methods were doing the opposite action of what was
intended.
Allowed keyboard actions again by regaining focus to frame.

Addition of the navigation buttons had stole the focus from
the frame to only the buttons, which prevented the keyboard
actions from being fired.
Fixed keyboard actions for next/previous.

The keyboard actions were reversed.