Linux: Support WebKitGTK backend

#include <QApplication>
#include <QWidget>
#include <QWindow>

extern "C" {
#include <gdk/gdkx.h>
#include <gtk/gtk.h>
#include <webkit2/webkit2.h>
}

int main(int argc, char *argv[]) {
	QApplication app(argc, argv);
	gtk_init(nullptr, nullptr);
	const auto window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
	const auto webview = webkit_web_view_new();
	webkit_web_view_load_uri(WEBKIT_WEB_VIEW(webview), "https://google.com");
	gtk_container_add(GTK_CONTAINER(window), GTK_WIDGET(webview));
	gtk_widget_show_all(window);
	QWidget::createWindowContainer(QWindow::fromWinId(gdk_x11_window_get_xid(gtk_widget_get_window(window))))->show();
	return app.exec();
}