Hide Saving Replay popup when no output is produced (fixes #420)

I.e. if the had Auto-Recording disabled and never pressed start, there won't be
any replays and it would just be the Done button, which is pointless.

src/main/java/com/replaymod/editor/gui/MarkerProcessor.java

@@ -53,6 +53,10 @@ public class MarkerProcessor {
         }
     }
 
+    public static boolean producesAnyOutput(ReplayFile replayFile) throws IOException {
+        return !getOutputSuffixes(replayFile).isEmpty();
+    }
+
     private enum OutputState {
         /** A new output file has begun but not data has been written yet. */
         NotYetWriting,