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)

Can we try changing the following to ```IEnumerable``` instead of ```ObservableCollection```? I think the sample file hierarchy, once constructed, will never change (i.e. that's why the tree view does not need to observe the ```Children``` collection). If changing the following to ```IEnumerable``` helps, then we change the data member ```childProperties``` to ```List<SampleFileEntry>``` (and also rename ```childProperties``` to ```childSampleFiles```).