introduce the qr-code-type `DCWEBRTC:[webrtc_instance]`.
dc_check_qr() returns this as the type DC_QR_WEBRTC
and these qr-codes can be passed to dc_set_config_from_qr() then;
this finally sets `webrtc_instance` using dc_set_config().
This uses the Fingerprint type more consistenly when handling
fingerprits rather then have various string representations passed
around and sometimes converted back and forth with slight differences
in strictness.
It fixes an important bug in the existing, but until now unused,
parsing behaviour of Fingerprint. It also adds a default length check
on the fingerprint as that was checked in some existing places.
Fially generating keys is no longer expensive, so let's not ignore
these tests.
