Create nav once

Current code creates the previous/next image navigation after each image loads. There are many issues with this:

1) An image might fail to load. No more navigation
2) Wasteful (code and cycles)
3) No one can modify / hook on the navigation items on `afterOpen` for example, as they will disappear
4) No one can add items to the dialog, these will be deleted with the `nextAll().remove()`.

This patch addresses all of these and is shorter (lighter)

Note: this builds on #17 and #16.