-Wincompatible-pointer-types warnings

We have a few warnings about implicit type casts with g_object_ref.

Context: https://blogs.gnome.org/chergert/2018/01/05/g_object_ref-and-wincompatible-pointer-types/

src/nautilus-files-view.c: In function ‘nautilus_files_view_init’:
src/nautilus-files-view.c:9525:47: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
     priv->toolbar_menu_sections->zoom_section = g_object_ref_sink (gtk_builder_get_object (builder, "zoom_section"));
                                               ^

src/nautilus-files-view.c:9526:51: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
     priv->toolbar_menu_sections->extended_section = g_object_ref_sink (gtk_builder_get_object (builder, "extended_section"));
                                                   ^

src/nautilus-properties-window.c: In function ‘schedule_group_change’:
src/nautilus-properties-window.c:1599:20: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
     change->window = g_object_ref (G_OBJECT (window));
                    ^

src/nautilus-properties-window.c: In function ‘schedule_owner_change’:
src/nautilus-properties-window.c:2020:20: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
     change->window = g_object_ref (G_OBJECT (window));
                    ^

src/nautilus-file.c: In function ‘nautilus_file_get_thumbnail_icon’:
src/nautilus-file.c:5491:15: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
         gicon = g_object_ref (pixbuf);
               ^

src/nautilus-progress-info-manager.c: In function ‘nautilus_progress_info_manager_constructor’:
src/nautilus-progress-info-manager.c:81:17: warning: return from incompatible pointer type [-Wincompatible-pointer-types]
         return g_object_ref (singleton);
                ~^~~~~~~~~~~~~~~~~~~~~~~~