feat: Make dc_msg_get_filename() return the original attachment filename (#4309)

It can be used e.g. as a default in the file saving dialog. Also display the original filename in
the message info. For these purposes add Param::Filename in addition to Param::File and use it as an
attachment filename in sent emails.

src/imex/transfer.rs

@@ -650,7 +650,9 @@ mod tests {
             _ => panic!("wrong chat item"),
         };
         let msg = Message::load_from_db(&ctx1, *msgid).await.unwrap();
+
         let path = msg.get_file(&ctx1).unwrap();
+        assert_eq!(path.with_file_name("hello.txt"), path);
         let text = fs::read_to_string(&path).await.unwrap();
         assert_eq!(text, "i am attachment");