No request to qrcode service when popover hidden

As [discussed](https://groups.google.com/forum/#!msg/re3-dev/cxEFYRW6z9Q/RQxcRoYrTLwJ) on the re3-dev mailing list we want to avoid triggering requests to the qrcodegenerator service, and filling the DB, when it's not necessary. Currently, after the popup has been shown once, any state change (e.g. zoom) triggers a request to the qrcodegenerator service, even when the popup is hidden. That's bad. This PR fixes that by triggering requests only when the popup is actually visible.

Please review.