implement gtk_window_set_titlebar

Makes it possible to use client-side decorations using GtkHeaderBar.
https://developer.gnome.org/gtk3/stable/GtkWindow.html#gtk-window-set-titlebar

Example:
```
let mut header_bar = gtk::HeaderBar::new().unwrap();
header_bar.set_title("GtkHeaderBar");
header_bar.set_close_button_show(true);
window.set_titlebar(&header_bar);
```

Please review carefully, I am pretty new to Rust.