treewide: Adapt to GtkHeaderBar API changes

https://docs.gtk.org/gtk4/migrating-3to4.html#adapt-to-gtkheaderbar-and-gtkactionbar-api-changes

- `show-close-button` replaced with `show_title_buttons` wherever
  applicable, respecting new default value of `TRUE`
- `title` removed and managed by setting window title if needed
    - NOTE: I'm not 100% sure if the title logic here matches the GTK3
      version. I also just removed the SIP account widget titles
      altogether, since it's I couldn't figure out where the widget gets
      placed.

Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/714>

src/calls-main-window.c

@@ -177,6 +177,7 @@ window_ussd_added_cb (CallsMainWindow *self,
   g_object_set_data_full (G_OBJECT (self->ussd_dialog), "ussd",
                           g_object_ref (ussd), g_object_unref);
   window_update_ussd_state (self, ussd);
+  gtk_window_set_title (GTK_WINDOW (self->ussd_dialog), _("USSD"));
   gtk_window_present (GTK_WINDOW (self->ussd_dialog));
 }