Store OpenSeadragon Viewer instantiation object handle

`OpenSeadragon.Viewer` instantiation object handle needs to be stored for attaching handlers to events like `open`, `close` etc.

Use case: To update current page number in Spotlight's multi-image display, `OpenSeadragon.Viewer` handle is needed for providing a callback to `page` event.

Reference: https://github.com/openseadragon/openseadragon/issues/317#issuecomment-32063089