Traewelling: replace legacy password login with OAuth2

This is a breaking change insofar as that traewelling support is no longer
provided automatically, but must be enabled by providing a traewelling.de
application ID and secret in travelynx.conf. However, as traewelling.de
password login is deprecated and wil soon be disabled, travelynx would break
either way. So we might or might not see travelynx 2.0.0 in the next days.

Automatic token refresh is still todo, but that was the case for password
login as well.

Closes #64