This logic is implemented similarly on other platforms, so we wish to
regain parity here.  Originally this behavior was implemented to always
fire an event with ModifiersState::empty(), but that was not the best as
it was not necessarily correct and could be a duplicate event.

This solution is perhaps the most elegant possible to implement the
desired behavior of sending a synthetic empty modifiers event when a
window loses focus, trading some safety for interoperation between the
NSWindowDelegate and the NSView (as the objc runtime must now be
consulted in order to acquire access to the ViewState which is "owned"
by the NSView).

Signed-off-by: Kristofer Rye <kristofer.rye@gmail.com>