Add support for Keys.RightControl/Keys.RightShift

Right now keyboard depends on CoreWindow.KeyUp/KeyDown events.
 The problem is that those events does not distinguish between
 LeftControl/RightControl (same for Shift,etc) but report a generic
 VirtualKey.Control.

In that case i check against CoreWindow.GetKeyState(...) and the _keys
 array to update the state of such keys.