st/password-entry: Fix crash when DConf changes after StEntry is destroyed

commit 8721c5db37 made StPasswordEntry
honor the 'disable-show-password' setting.

Unfortunately, it introduced a lifecycle bug where the signal handler
for noticing setting changes can out live the entry itself.

This commit fixes the problem by using g_signal_connect_object

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2004>

src/st/st-password-entry.c

@@ -37,6 +37,7 @@ struct _StPasswordEntry
 struct _StPasswordEntryPrivate
 {
   ClutterActor *peek_password_icon;
+
   gboolean      password_visible;
   gboolean      show_peek_icon;
 };
@@ -229,10 +230,11 @@ st_password_entry_init (StPasswordEntry *entry)
 
   st_password_entry_set_show_peek_icon (entry, TRUE);
 
-  g_signal_connect (st_settings_get (),
+  g_signal_connect_object (st_settings_get (),
                            "notify::disable-show-password",
                            G_CALLBACK (on_disable_show_password_changed),
-                    entry);
+                           entry,
+                           0);
 
   clutter_text = st_entry_get_clutter_text (ST_ENTRY (entry));
   clutter_text_set_password_char (CLUTTER_TEXT (clutter_text), BLACK_CIRCLE);