Accept shift/ctrl keycodes.

src/Windows/Perspex.Win32/Input/KeyInterop.cs

@@ -195,6 +195,8 @@ namespace Perspex.Win32.Input
             { 9, Key.Tab },
             { 12, Key.Clear },
             { 13, Key.Return },
+            { 16, Key.LeftShift},
+            { 17, Key.LeftCtrl},
             { 18, Key.LeftAlt },
             { 19, Key.Pause },
             { 20, Key.Capital },