Update lightbox_dialog.js - Adding an event for DOM change when modal is already shown

There is an event for when the modal is new and the DOM is ready for use (by using the "show" event in bootstrap-modal.js).
There is no event for when the DOM has changed and the modal already exists.

Such event was added in this commit.
Merge pull request #581 from eranhirs/patch-1

Update lightbox_dialog.js - Adding an event for DOM change when modal already exists.