DatePicker and expand-a-node in TreeView: updates to the Bag-O-Tricks for the July CTP

I promised you I'd get these in "this week". I guess, depending on how you split the weeks, I delivered.

Most of the details are in the post on the Community Site for WPF.

DatePicker/MonthCalendar were implemented by our friends in the Advanced Technology Center in Beijing. Thanks, guys.

I added a helper class to the Library assembly that helps with common operations on a TreeView. Let me know if it works out for ya.

Hopefully I'll have another late night this week where I can hack on samples. I still have some very cool stuff waiting to be factored and posted.