Contact::get_color() should use contact's key fingerprint instead of email address

[iequidoo]

This fits into the further plans for adding multi-transport messaging, but can be done already now. Currently, if the contact e.g. AEAP-moves to another address, its color changes. So, the suggestion is:

• Use the fingerprint returned by Peerstate::peek_key_fingerprint(false), if any. It can return an unverified Autocrypt key fingerprint, but i think that's fine, maybe even good if the color changes if the contact verification breaks.
• Otherwise use the email address as currently.