Created GuiMarkerTimeline to display Markers again

Created helper methods in ReplayHandler to provide information about MarkerKeyframes

src/main/java/eu/crushedpixel/replaymod/holders/MarkerKeyframe.java

@@ -5,7 +5,7 @@ import lombok.Data;
 
 @Data
 @AllArgsConstructor
-public final class MarkerKeyframe {
+public class MarkerKeyframe {
 
     private int realTimestamp;
     private Position position;