Improve the navigation within the flight wizard

[István Váradi]

As the possible routes within the flight wizard get more complex, one should be able to jump between pages using IDs.

Therefore each page should be given an ID, and jumpPage() should accept such an ID as well.

It should also be possible to set the ID of the default next page for a page and nextPage() should use that, if present.

[István Váradi <ivaradi@…>]

In 8547ad78cc291acc0f1b850ead210cac23736890:

The pages now have a textual identifier and can be referenced with it (re #286)