PlaybackManager: add getSheetDurationInMs() method

src/Playback/PlaybackManager.ts

@@ -1253,6 +1253,11 @@ export class PlaybackManager implements IPlaybackParametersListener {
         }
     }
 
+    /** Returns the duration of the piece in ms. Including repeats. */
+    public getSheetDurationInMs(): number {
+        return this.timingSource.getDurationInMs(this.musicPartManager.MusicSheet.SheetEndTimestamp);
+    }
+
         //private class MidiChannelInfo
         //{
         //    public List<IInstrument> subscribers = new List<IInstrument>();