save subject for messages (#2274)

save subject for messages:

- new api `dc_msg_get_subject()`,

- when quoting, use the subject of the quoted message as the new subject, instead of the
last subject in the chat

src/ephemeral.rs

@@ -311,7 +311,7 @@ pub(crate) async fn delete_expired_messages(context: &Context) -> Result<bool, E
             // If you change which information is removed here, also change MsgId::trash() and
             // which information dc_receive_imf::add_parts() still adds to the db if the chat_id is TRASH
             "UPDATE msgs \
-             SET chat_id=?, txt='', txt_raw='', mime_headers='', from_id=0, to_id=0, param='' \
+             SET chat_id=?, txt='', subject='', txt_raw='', mime_headers='', from_id=0, to_id=0, param='' \
              WHERE \
              ephemeral_timestamp != 0 \
              AND ephemeral_timestamp <= ? \