Restructures Escalator method hierarchy (#13334)

Previously AbstractRowContainer.paintInsertRows was simply overridden by
BodyRowContainer, which was hard to follow. Now ARC.pIR is an abstract method
and the implementation has been put into another protected method.

Change-Id: Ia47a46216d2f9713b958785b5239d6850b1ede0d

client/src/com/vaadin/client/ui/grid/Escalator.java

 package com.vaadin.client.ui.grid;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.LinkedList;
 import java.util.List;
          *            the number of rows to insert
          * @return a list of the added row elements
          */
-        protected List<TableRowElement> paintInsertRows(final int visualIndex,
-                final int numberOfRows) {
+        protected abstract void paintInsertRows(final int visualIndex,
+                final int numberOfRows);
+
+        protected List<TableRowElement> paintInsertStaticRows(
+                final int visualIndex, final int numberOfRows) {
             assert isAttached() : "Can't paint rows if Escalator is not attached";
 
             final List<TableRowElement> addedRows = new ArrayList<TableRowElement>();
 
             Profiler.leave("Escalator.AbstractStaticRowContainer.refreshRows");
         }
+
+        @Override
+        protected void paintInsertRows(int visualIndex, int numberOfRows) {
+            paintInsertStaticRows(visualIndex, numberOfRows);
+        }
     }
 
     private class HeaderRowContainer extends AbstractStaticRowContainer {
         }
 
         @Override
-        protected List<TableRowElement> paintInsertRows(final int index,
-                final int numberOfRows) {
+        protected void paintInsertRows(final int index, final int numberOfRows) {
             if (numberOfRows == 0) {
-                return Collections.emptyList();
+                return;
             }
 
             /*
                 fireRowVisibilityChangeEvent();
                 sortDomElements();
             }
-            return addedRows;
         }
 
         /**
 
             if (escalatorRowsNeeded > 0) {
 
-                final List<TableRowElement> addedRows = super.paintInsertRows(
+                final List<TableRowElement> addedRows = paintInsertStaticRows(
                         index, escalatorRowsNeeded);
                 visualRowOrder.addAll(index, addedRows);