feat: implement clear all feature

Close #75

Author: hy8246

src/main/java/com/flowingcode/vaadin/addons/twincolgrid/TwinColGrid.java

@@ -270,7 +270,11 @@ public void addRightGridClassName(String classname) {
   public void removeRightGridClassName(String classname) {
     getRightGrid().removeClassName(classname);
   }
-
+  
+  public void clearAll() {
+    updateSelection(new HashSet<>(), new HashSet<>(right.getItems()));
+  }
+  
   private void setDataProvider(ListDataProvider<T> dataProvider) {
     leftGridDataProvider = dataProvider;
     getLeftGrid().setDataProvider(dataProvider);