Fix menu items not working on Windows

Dave,

I think this may be a slightly better fix than your d57914c.

You have put the `event.Skip()` in an `ifdef __WXOSX__`, which *does* solve this particular issue, but I think it may be better to tackle the root cause of the problem: `event.Skip()` should be called *if, and only if, we have not handled the event ourselves*.

This commit does that by moving the call to `event.Skip()` into the `default` block of the `switch`.

Let me know what you think.
Caesar