Tutorial/Documentation should always be opened in the first iframe

[Akron]

When plugins are introduced with iframes, the wrong iframe may be considered in tutorial.js for retrieving the correct pathname.

https://github.com/KorAP/Kalamar/blob/master/dev/js/src/tutorial.js#L197

This can lead to weird CORS related errors.

One solution may be to open the tutorial always as a first iframe, or store the tutorial iframe in a global variable.