api: Add dc_msg_save_file() which saves file copy at the provided path (#4309)

... and fails if file already exists. The UI should open the file saving dialog, defaulting to
Downloads and original filename, when asked to save the file. After confirmation it should call
dc_msg_save_file().

src/imex/transfer.rs

@@ -656,6 +656,12 @@ mod tests {
         let text = fs::read_to_string(&path).await.unwrap();
         assert_eq!(text, "i am attachment");
 
+        let path = path.with_file_name("saved.txt");
+        msg.save_file(&ctx1, &path).await.unwrap();
+        let text = fs::read_to_string(&path).await.unwrap();
+        assert_eq!(text, "i am attachment");
+        assert!(msg.save_file(&ctx1, &path).await.is_err());
+
         // Check that both received the ImexProgress events.
         ctx0.evtracker
             .get_matching(|ev| matches!(ev, EventType::ImexProgress(1000)))