use G_DEFINE_TYPE-supplied ev_sidebar_thumbnails_parent_class variable,

use G_DEFINE_TYPE-supplied ev_sidebar_thumbnails_parent_class variable,

* shell/ev-sidebar-thumbnails.c (ev_sidebar_thumbnails_destroy)
(ev_sidebar_thumbnails_class_init): use G_DEFINE_TYPE-supplied
ev_sidebar_thumbnails_parent_class variable, don't define another
parent_class variable.

* shell/ev-page-action.c (connect_proxy, ev_page_action_finalize)
(ev_page_action_class_init): ditto.

* pixbuf/pixbuf-document.c (pixbuf_document_finalize)
(pixbuf_document_class_init): ditto.

* backend/ev-link.c (ev_window_dispose, ev_link_class_init):
ditto.