Ensure that application windows never show menu bars

Before commit f79285bb running Epiphany on a desktop environment which hints to applications that there is no top bar where the appmenu would go, the application icon would be added to the header bar by GTK+, and the appmenu could be triggered clicking it. This would be the case for e.g. Budgie or XFCE.

After the appmenu/winmenu merger, the menu associated to the application icon is empty, and each window has an almost-empty menu bar with a single “Web” menu, which has no entries inside. This is not only jarring: it is completely useless.

This sets the GtkApplicationWindow::show-menubar property to FALSE in order to fix the second offense.