macos: allow dropping NSEvent without propagation

macos: allow dropping NSEvent without propagation

There are cases we might want to consume a NSEvent without creating a
GdkEvent or passing it along to the NSApplication for processing. This
creates a new value we can use and check against to propagate that without
having to do out parameters at the slightly odd invalid pointer value for
a GdkEvent (similar to how MMAP_FAILED is done).