title: Update ListPicker on SelectionChanged
body: <p>I have two list pickers. When first list picker have selection changed it should filter data of second list picker. 
  Second ListPicker xaml is like this</p>
  
  <pre><code> &lt;TextBlock HorizontalAlignment="Left" Style="{StaticResource PhoneTextNormalStyle}" TextWrapping="Wrap" Text="text" VerticalAlignment="Bottom"/&gt;
                  &lt;toolkit:ListPicker VerticalAlignment="Bottom" Name="CategoryList" ItemsSource="{Binding TransactionCategories, Mode=TwoWay}" SelectedItem="{Binding SelectedTransactionCategory, Mode=TwoWay}"&gt;
                  &lt;toolkit:ListPicker.ItemTemplate&gt;
                      &lt;DataTemplate&gt;
                          &lt;StackPanel Orientation="Horizontal"&gt;
                              &lt;Rectangle Fill="Red" Width="24" Height="24"/&gt;
                              &lt;TextBlock Text="{Binding CategoryName,Mode=TwoWay}" Margin="12 0 0 0"/&gt;
                          &lt;/StackPanel&gt;
                      &lt;/DataTemplate&gt;
                  &lt;/toolkit:ListPicker.ItemTemplate&gt;
                  &lt;toolkit:ListPicker.FullModeItemTemplate&gt;
                      &lt;DataTemplate&gt;
                          &lt;StackPanel Orientation="Horizontal" Margin="16 21 0 20"&gt;
                              &lt;Rectangle Width="43" Height="43"/&gt;
                              &lt;TextBlock Text="{Binding CategoryName, Mode=TwoWay}" Margin="16 0 0 0" FontSize="43" FontFamily="{StaticResource PhoneFontFamilyLight}" TextWrapping="Wrap"/&gt;
                          &lt;/StackPanel&gt;
                      &lt;/DataTemplate&gt;
                  &lt;/toolkit:ListPicker.FullModeItemTemplate&gt;
              &lt;/toolkit:ListPicker&gt;
  </code></pre>
  
  <p>First picker fires the event GroupCategoryList_SelectionChanged</p>
  
  <pre><code> private void GroupCategoryList_SelectionChanged(object sender, SelectionChangedEventArgs e)
      {
          var category = (TransactionGroupCategory) GroupCategoryList.SelectedItem;
          ((TransactionEditViewModel)DataContext).FilterCategoryByGroup(category.GroupCategoryId);
      }
  </code></pre>
  
  <p>This is a metod in ViewModel</p>
  
  <pre><code> public  void FilterCategoryByGroup(int groupCategoryId)
      {
  
          TransactionCategories = ToObservableCollection(DatabaseBl.GetData&lt;TransactionCategory&gt;().Where(x =&gt; x.GroupCategoryId == groupCategoryId).OrderByDescending(tc =&gt; tc.TransactionCount));
          if (TransactionCategories.Count &gt; 0)
              SelectedTransactionCategory = TransactionCategories[0];
      }
  </code></pre>
  
  <p>The second list picker never get refreshed in UI, even though new data was fetched through filter method. What could be wrong with this. Binding issue maybe? </p>