Set subject to Re: <last subject> for better compability with normal MUAs

The code in dc_receive_imf.rs looks a bit funny, an alternative would be a function:

fn upcate_chat_last_subject(context: &Context, chat_id: &ChatId, mime_parser: &mut MimeMessage) -> Result<()> {
    let mut chat = Chat::load_from_db(context, *chat_id)?;
    chat.param.set(Param::LastSubject, mime_parser.get_subject().ok_or_else(||Error::Message("No subject in email".to_string()))?);
    chat.update_param(context)?;
    Ok(())
}

src/param.rs

@@ -103,6 +103,9 @@ pub enum Param {
     /// For Chats
     Selftalk = b'K',
 
+    /// For Chats: So that on sending a new message we can sent the subject to "Re: <last subject>"
+    LastSubject = b't',
+
     /// For Chats
     Devicetalk = b'D',