Change start page's samples list box to treeView

Added show samples in folder
![capture](https://cloud.githubusercontent.com/assets/8348435/4116505/c1009dda-3285-11e4-8b33-550f9b37465a.PNG)

No, it has already been declared in XAML, unless we need to access this tree view outside of start page (and I don't think anyone else should be accessing the tree view outside of start page. It belongs to start page only.