Allow usage of custom grid

[stefanuebe]

Currently the model uses an instance of GridEx, which seems not to add any additional functionality. We would like to use a SelectionGrid instead (or some other custom grid, which provides additional functionality).

To allow that, the simples solution would be to use the normal Grid instead of GridEx and add a "createGrid" method to the base class, which can be overridden by any subclass.

Something like this:

  public TwinColGrid(final ListDataProvider<T> dataProvider, String caption) {
    available = new TwinColModel<>(createGrid(true), "twincol-grid-available");
    selection = new TwinColModel<>(createGrid(false),"twincol-grid-selection");
    ...
  }

  protected Grid<T> createGrid(boolean available) {
     return new GridEx<>();
  }

    private static final class TwinColModel<T> implements Serializable {
      final Grid<T> grid;
      ...

      TwinColModel(String className, Grid<T> grid) {
         this.grid = grid;
      }

[javier-godoy]

Makes sense.

[javier-godoy]

Currently the model uses an instance of GridEx, which seems not to add any additional functionality

GridEx is used for "publishing" the protected method createSortingComparator which is required in order to support ordered values (#66) because this addon is compatible with Vaadin 14+ and thus we cannot use ListDataView. I'll duplicate the code from Grid::createSortingComparator as that preserves the current behavior. (createSortingComparator is not final, thus it's possible that a subclass of Grid want to override it, although it doesn't seem likely).

Add a "createGrid" method to the base class, which can be overridden by any subclass.

That approach entails calling overridable methods from the constructor (see https://stackoverflow.com/a/3404369/1297272).
Instead, I'll add a constructor that receives a Supplier<Grid<T>> (the same supplier instantiates both grids) and another constructor that receives two Grid<T> instances (one for each side).

[javier-godoy]

Released in v2.7.0