Added a background loader, periods will now be loaded in the background. Left blue button wil now pauze / resume the period switcher

LangsteHittegolfPeriode.java

@@ -13,5 +13,8 @@ public class LangsteHittegolfPeriode extends Grootheid{
         setPeriod(StatisticsCalculator.langsteHittegolfPeriode(list));
     }
     
-
+    public void displayGraph()
+    {
+    }
+    
 }