MacOS: Implement EventLoopExtPumpEvents and EventLoopExtRunOnDemand

The implementation of `pump_events` essentially works by hooking into the
`RunLoopObserver` and requesting that the app should be stopped the next time
that the `RunLoop` prepares to wait for new events.

Originally I had thought I would poke the `CFRunLoop` for the app directly and
I was originally going to implement `pump_events` based on a timeout which I'd
seen SDL doing.

I found that `[NSApp run]` wasn't actually being stopped by asking the RunLoop
to stop directly and inferred that `NSApp run` will actually catch this and
re-start the loop.

Hooking into the observer and calling `[NSApp stop]` actually seems like a
better solution that doesn't need a hacky constant timeout.

The end result is quite similar to what happens with existing apps that
call `run_return` inside an external loop and cause the loop to exit for
each iteration (that also results in the `NSApp` stopping each
iteration).

src/platform/mod.rs

@@ -11,8 +11,8 @@
 //!
 //! And the following platform-specific modules:
 //!
-//! - `run_ondemand` (available on `windows`, `android`)
-//! - `pump_events` (available on `windows`, `android`)
+//! - `run_ondemand` (available on `windows`, `macos`, `android`)
+//! - `pump_events` (available on `windows`, `macos`, `android`)
 //! - `run_return` (available on `windows`, `unix`, `macos`, and `android`)
 //!
 //! However only the module corresponding to the platform you're compiling to will be available.
@@ -34,10 +34,10 @@ pub mod windows;
 #[cfg(x11_platform)]
 pub mod x11;
 
-#[cfg(any(windows_platform, android_platform))]
+#[cfg(any(windows_platform, macos_platform, android_platform))]
 pub mod run_ondemand;
 
-#[cfg(any(windows_platform, android_platform,))]
+#[cfg(any(windows_platform, macos_platform, android_platform,))]
 pub mod pump_events;
 
 #[cfg(any(