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.

Two more problem with those events are that they don't report ALT key,
and might also interpet mouse/touch gestures and fire the equavent key
sequence.