Start to add a itinerary bundle file format

This is needed for transferring multiple reservations and associated
documents (pkpass, PDFs, emails, etc) in an atomic way, without losing
their associations and meta data. We already now have problems getting
stuff from KMail into the app via KDE Connect, and things will get much
more complicated once we support attaching random documents as well.

Additionally, this will give as an easy way to export (and import) the
entire data set of the app, which is particularly important on Android,
where there is no other way to backup the data without root access.