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)

Please download this [Editor Guidelines plug-in](http://visualstudiogallery.msdn.microsoft.com/0fbf2878-e678-4577-9fdb-9030389b338c/) for Visual Studio, and put two guidelines at 80 and 100 characters. Once the code goes much longer than 80 characters, then we should try to break it into two lines. If it reaches 100 characters, then it must be broken into two lines for readability.

For example, I would change the following lines to:

```cs
var dp = e.OriginalSource as DependencyObject;
var treeViewItem = VisualUpwardSearch<TreeViewItem>(dp) as TreeViewItem;
if (treeViewItem != null)
    treeViewItem.IsExpanded = !treeViewItem.IsExpanded;
```

Few points to note:

1. Don't put ```if``` statement and its body on one line.
2. Use ```var``` instead of the type name for bigger types (e.g. ```DependencyObject```).