gtk-4 generated builders should panic if GTK hasn't been initialized

[anna-hope]

Currently, trying to use some builders before initializing GTK can cause a segfault. Instead, the generated builder should check if GTK has been initialized, and panic if not.

Minimal example

use gtk4::prelude::*;
use gtk4::{Application, ApplicationWindow, glib};

const APP_ID: &str = "me.annahope.gtk4-rs-segfault";

fn build_ui(app: &Application) {
    let window = ApplicationWindow::builder()
        .application(app)
        .title("Segfault")
        .build();  // Segfaults if GTK hasn't been initialized.

    window.present();
}

fn main() -> glib::ExitCode {
    let app = Application::builder().application_id(APP_ID).build();
    // app.connect_activate(build_ui);
    build_ui(&app); // Segfaults.
    app.run()
}

I tried fixing this naively by adding a check to the builder codegen code anna-hope@a3ee252, but then realized that we definitely can't do this for all builders because:

• Some builders, like https://github.com/gtk-rs/gtk4-rs/blob/8850a124cccded5f9b7483c88b1891cafe30e5f9/gtk4/src/auto/application.rs#L103 actually can/need to be called before GTK is initialized
• Others, like https://github.com/gtk-rs/gtk4-rs/blob/8850a124cccded5f9b7483c88b1891cafe30e5f9/gdk4/src/auto/cursor.rs#L140 aren't even part of gtk4 itself, so it doesn't make sense for them to have this check (and they can't do it anyway)

I briefly investigated the builder_properties to see if we could set some field there, but couldn't find an easy way to add one. I could look into it further, but I am brand new to this project and GTK overall, so perhaps there is a better way to accomplish this that I don't know about.

[bilelmoussaoui]

The solution is actually pretty easy, doesn't require anything like you did. Just add assert_initialized_main_thread!(); macro call there.