initialize/dispose tooltips and legends from core.

fixes a new issue introduced with the new changes, because the older versions of tooltips where disposed/created evetime the data changed, now tooltips are cached and only updated when necessary,  we need to handle the creation/diposing when the control is loaded/disposed.

src/LiveChartsCore/Chart.cs

@@ -298,6 +298,8 @@ public virtual void Load()
     {
         IsLoaded = true;
         _isFirstDraw = true;
+        View.Tooltip = LiveCharts.DefaultSettings.GetTheme().DefaultTooltip();
+        View.Legend = LiveCharts.DefaultSettings.GetTheme().DefaultLegend();
         Update();
     }
 
@@ -307,6 +309,8 @@ public virtual void Load()
     public virtual void Unload()
     {
         IsLoaded = false;
+        View.Tooltip = null;
+        View.Legend = null;
         _everMeasuredElements.Clear();
         _toDeleteElements.Clear();
         _activePoints.Clear();