Reverted change to Virutalizing ComboBox because it breaks grouping

Last change set the itemspanel to virtualizing but this breaks the grouping behaviour of the combobox:

With Virtualizing:
![2013-05-24 13-19-24_](https://f.cloud.github.com/assets/3532342/560022/0a3646b2-c464-11e2-97ea-367454ef5dc0.png)

Without Virtualizing:
![2013-05-24 13-20-21_](https://f.cloud.github.com/assets/3532342/560023/0a4bd0a4-c464-11e2-8738-263b6bd602c1.png)

If there is need for a virtualizing ComboBox simply use:

 ```
<ComboBox.ItemsPanel>
    <ItemsPanelTemplate>
        <VirtualizingStackPanel />
    </ItemsPanelTemplate>
</ComboBox.ItemsPanel>
 ```

Or make an seperate style.