Add CLI argument parsing example

[REALERvolker1]

Hey there. I am making a GTK application, and I would like to take command line arguments.

I can run a GTK application that ignores command line arguments that are passed, my code for this is

let app = gtk4::Application::builder()
    .application_id("com.github.REALERvolker1.example")
    .build();
app.run_with_args::<glib::GString>(&[]);

However, I would like to learn how to use GTK's built-in argument parsing, hopefully removing some dependencies. I know that it is definitely harder than clap or rolling my own custom one (requiring me to either hardcode or macro a bunch of stuff into oblivion), but I would like to try it.

I checked out the GTK documentation, and it seems like handle-local-options is the best way to do this. It won't give me as much control, but I would prefer to have the help page formatting from gtk, even if it does include the --help-all and --help-gapplication flags. I don't know if this is how that works, please do correct me if I'm wrong. I would like to use the nice layout that comes with this, and keep everything nice and structured. The help text requirement is also really nice imo.

I found that I have to add a bunch of gio::ActionEntry to the application. However, I don't know how that translates into CLI argument parsing, and how that interacts with the app.connect_command_line(|app, cli| {/* ... */}); handler.

I have seen the methods on the ApplicationCommandLine class, and it looks very featureful. Having an arg parsing example would really make this a lot easier to learn.

There is definitely interest in this see (#730), and it would also clear up some confusion with gtk weirdness (#1436, #1039)