Add platform channel System.exitApplication and System.requestAppExit support

[gspencergoog]

Description

This adds support for System.exitApplication and sending and handling the response from System.requestAppExit to the platform channel.

It also introduces a FlutterApplication class that is a subclass of NSApplication, and overrides the terminate message so that it can ignore it if the framework decides to cancel the termination, and to allow for async communication with the framework before termination.

If the application doesn't use a FlutterApplication as its NSPrincipalClass in Info.plist for the application, then it all works as before: the application will be unable to cancel the request. In debug mode it does log once per run, if the application is accessed, that some capabilities are disabled (like exitApplication). I'm not really sure if we should be doing the log message. It seems like a good prompt to remind people to migrate, and how, but I could easily be convinced that it's log spam.

Paired with flutter/flutter#121378 to enable the framework to respond properly. Before that lands, if anyone switches to use FlutterApplication as their principal class, termination requests will go unanswered.

Related Issues

• Add window/application lifecycle delegation for desktop shells flutter#30735

Tests

• Adds tests for the round trip from framework request for exit, to application termination request from the engine to framework, and then a response to the engine to actually terminate.

[gspencergoog]

Ok, PTAL: I've added a test that goes the round trip: handles a call to exit the application from the framework, asks the framework if that's OK (so that if other listeners are registered, they get asked), and then either exits or doesn't depending on the response. If it had come from the OS, then it would just do the last part (query the framework and then possibly exit).

The only thing I couldn't figure out how to test was the actually call to [[FlutterApplication sharedApplication] terminate:nil], since that requires that there be a real application running (making a mock subclass would just bypass the thing I'm trying to test). There's only a couple of lines of code there.

I was able to cut down the necessary additions to FlutterAppDelegate to be just an internal property that is the termination handler.

The termination handler is an internal class, and so is the property on FlutterAppDelegate for now. I considered making them public, but I'd like to see a use case first.

[cbracken]

A couple quick comments before I grab dinner. Will finish the rest of the review in a bit!

[cbracken]

lgtm!