macos: Send dragging events directly to the display

macos: Send dragging events directly to the display

Instead of adding events to the application event queue, dispatch
them directly to the right display. We know this when the event is
to be dispatched.

This is the same as used for the `sendEvent` method in `GdkMacosWindow`.

To achieve this I factored out the generic NSEvent to GdkEvent translation.
We can send an event directly, when we receive it in the GdkMacosWindow
directly from the OS.