Add platform channel System.exitApplication and System.requestAppExit support

[robert-ancell]

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

Fixes Linux part of flutter/flutter#30735

[flutter-dashboard]

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[robert-ancell]

@gspencergoog I haven't tested this - can you confirm this is using the API as intended?

[robert-ancell]

I've started platform plugin tests in #39992 - will land that first to use that framework.

[gspencergoog]

Yes, this all looks right now.

[gspencergoog]

[chinmaygarde]

From Triage: May I land this?

[gspencergoog]

I think not yet. Robert's still working out a segfault in the engine. Right @robert-ancell ?

[robert-ancell]

Yes, I wanted to be sure the crash wasn't due to this change first.

[robert-ancell]

Crash now fixed, will land once the tests complete.

[Kingtous]

This line totally blocks the additional listener provided by plugins, such like window_manager. :(

[gspencergoog]

Yes, I agree, it does, but the alternative is returning FALSE, which would not give the app a chance to stop the exit when the last window goes away.

Perhaps this should only make the exit request and return TRUE if there is only one window open. That would still block the plugin listeners on the last window, though.

[iciclize]

I found the change prevents *_plugin_dispose(GObject* object) from being called.

The some_plugin_dispose in the generated template will never be called.

static void some_plugin_dispose(GObject* object) {
  G_OBJECT_CLASS(some_plugin_parent_class)->dispose(object);
}

static void some_plugin_class_init(SomePluginClass* klass) {
  G_OBJECT_CLASS(klass)->dispose = some_plugin_dispose;
}