Build with /W4 (#197)

All projects are built with warning level 4 (/W4) and treat warnings as errors (/WX).
Fixed build errors resulting from enabling these compiler flags.

src/Calculator/Views/Calculator.xaml.cpp

@@ -277,7 +277,7 @@ void Calculator::OnLayoutStateChanged(_In_ Object^ sender, _In_ Object^ e)
     UpdatePanelViewState();
 }
 
-void Calculator::OnIsStandardPropertyChanged(bool /*oldValue*/, bool newValue)
+void Calculator::OnIsStandardPropertyChanged(bool /*oldValue*/, bool /*newValue*/)
 {
     UpdateViewState();
     UpdatePanelViewState();