Start locate-keys when a recipient is added
* src/mail.cpp (Mail::locate_keys): Check for recipient keys. (do_locate): New. (Mail::needs_crypto): Return flags. * src/mail.h: Update accordingly. * src/mailitem-events.cpp (EVENT_SINK_INVOKE): Handle recipient changes and do locate_keys if necessary.
Checked addresses are stored in a set so that they are not
searched twice per session to avoid spammy requests.
This feature is mostly useful for users of auto-key-locate
where this can be used to retrieve keys from dane etc.