Simplify the section header creation, changing it so that the header is
created in init() and destroyed in dispose() and tie the header
lifetime to the section lifetime. This makes the object lifetime much
easier to understand and fixes various criticals removing the last row
(which triggers header destruction) that appeared after commit 1af00154.

15:06:04:0981 GLib-GObject invalid unclassed pointer in cast to 'GsProgressButton'
15:06:04:0982 Gtk gtk_css_provider_load_from_data: assertion 'GTK_IS_CSS_PROVIDER (css_provider)' failed
15:06:04:0982 Gtk gtk_stack_set_visible_child_name: assertion 'GTK_IS_STACK (stack)' failed
15:06:04:0982 GLib-GObject invalid unclassed pointer in cast to 'GtkWidget'
15:06:04:0982 Gtk gtk_widget_show: assertion 'GTK_IS_WIDGET (widget)' failed
15:06:04:0982 GLib-GObject invalid unclassed pointer in cast to 'GtkButton'
15:06:04:0982 Gtk gtk_button_set_label: assertion 'GTK_IS_BUTTON (button)' failed