Draft: Improve template overrides

Proposed API Additions

namespace Gtk {
    export class Widget {
        static set_template_from_uri(uri: string): void;
        
        // Potential override to simplify usage with static fields
        get_template_child<T extends GObject.Object>(constructorOrName: string | Function, name?: string): T;
        get_template_child<T extends GObject.Object>(constructor: Function, name: string): T;
        get_template_child<T extends GObject.Object>(name: string): T;

        /**
         * @example
         * class Example {
         *     _children = this.get_template_children();
         * }
         */
        get_template_children<T extends Record<string, GObject.Object>>(): T;
    }
}