Passive Grouping+Virtualization in ComboBox

Added ComboBoxHelper that has an extension method that enables virtualization and grouping in .NET 4.5 and will do nothing in .NET 4.0.

Fixes #596

![m](https://f.cloud.github.com/assets/251501/1366935/07628c46-38f1-11e3-811a-61c8d215b1c7.PNG)