Keyboard navigation with arrows malfunctioning when Lightbox initialized multiple times

[MichalRemis]

When lightbox is initialized multiple times (e.g. when Ajax content is loaded and lightbox needs to be reenabled), Lightbox keyboard navigation doesn't work properly - arrow keys are skipping multiple images, not just one. It's because keyboard listeners are bound multiple times when opening lightbox.

Example:

1. go to https://lokeshdhakar.com/projects/lightbox2/ ,
2. open console and type lightbox.init()
3. click on FOUR IMAGE SET lightbox and try to navigate with arrows