Grouping insertion adorner fix

this pr should fix a painting bug of the insertion adorner if we have a grouped listbox.

![2014-09-25_21h44_55](https://cloud.githubusercontent.com/assets/658431/4411178/7eac0de2-44ec-11e4-8417-3daa1cf89d8f.png)

the adorner should paint after the latest item in a group and not before the first item in the next group.

![2014-09-25_21h34_13](https://cloud.githubusercontent.com/assets/658431/4411090/d7df6dd8-44eb-11e4-8b59-d86e867ec3a3.png)