Fix phong material binding issue.
Fix phong material binding issue.

Ref: https://social.msdn.microsoft.com/Forums/en-US/5e42e6a1-740f-48b2-9908-09a474f9d413/error-converter-failed-to-convert-value-of-type-when-type-is-derived?forum=winappswithcsharp

Add phong material to Theme.xaml to solve this issue
Iluvatar82/helix-toolkit,helix-toolkit/helix-toolkit,JeremyAnsel/helix-toolkit,chrkon/helix-toolkit,holance/helix-toolkit