examples/tutorials: Use NSApp terminate() instead of sending an event

I discovered it can be done this way while working on the C-side API for the same purpose - see gstreamer!3532 (merged). Looks much less confusing and has the same effect.

Unfortunately the crate we're using doesn't yet have bindings for this particular function, so I used objc::msg_send! directly. Using the cacao crate where terminate() is present was also an option, but their repo says it's rather 'beta quality' so I don't think that'd be a good idea to use it in examples.