Add XEP-0280 (Carbons) support for pidgin-gpg.c

Encrypt to main key (if selected),
use "to" instead of "from" for carbonated sent messages to identify the correct communication partner.
Update pidgin-gpg.c

Fixed issue for forwarded chat markers and receipts that were otherwise disabling encryption after every carbonated message sent from Conversations.
(received <message> without <x> always disabled encryption)
Also it might be better from a security perspective to not just fall back to plain text as soon as we receive an unencrypted message-
Update pidgin-gpg.c

Forgot sender_key declaration..
Merge pull request #1 from github-k8n/master

Merge from github-8kn:master
- Be more forgiving if PREF_MY_KEY is missing.
Merge pull request #2 from Draghtnod/github-8kn

Merge with github-8kn branch