Parameterize client-side Grid.addColumn to return the argument type (#16262)

This allows invoking custom methods on the return value.

Change-Id: I5f2749b792db3627b46e676aee82dc672fec7113

client/src/com/vaadin/client/widgets/Grid.java

      *            the column to add
      * @return given column
      */
-    public Column<?, T> addColumn(Column<?, T> column) {
+    public <C extends Column<?, T>> C addColumn(C column) {
         addColumn(column, getColumnCount());
         return column;
     }
      *             if Grid's current selection model renders a selection column,
      *             and {@code index} is 0.
      */
-    public Column<?, T> addColumn(Column<?, T> column, int index) {
+    public <C extends Column<?, T>> C addColumn(C column, int index) {
         if (column == selectionColumn) {
             throw new IllegalArgumentException("The selection column many "
                     + "not be added manually");

uitest/src/com/vaadin/tests/widgetset/client/grid/GridColumnAutoWidthClientWidget.java

     }
 
     private Col addColumn(String header) {
-        Col column = (Col) grid.addColumn(new Col(header));
+        Col column = grid.addColumn(new Col(header));
         grid.getHeaderRow(0).getCell(column)
                 .setHtml("<span>" + header + "</span>");
         return column;