gcal-utils: Combine GNOME Settings launchers

This refactor combines the functions
gcal_utils_launch_online_accounts_panel and
gcal_utils_launch_datetime_settings_panel into a single function that
takes a panel ID as an argument.

src/gui/calendar-management/gcal-edit-calendar-page.c

@@ -259,9 +259,9 @@ on_settings_button_clicked_cb (GtkWidget            *button,
   goa = e_source_get_extension (parent, E_SOURCE_EXTENSION_GOA);
 
   app = g_application_get_default ();
-  gcal_utils_launch_online_accounts_panel (g_application_get_dbus_connection (app),
-                                           e_source_goa_get_account_id (goa),
-                                           NULL);
+  gcal_utils_launch_gnome_settings (g_application_get_dbus_connection (app),
+                                    "online-accounts",
+                                    e_source_goa_get_account_id (goa));
 
   GCAL_EXIT;
 }

src/gui/calendar-management/gcal-new-calendar-page.c

@@ -513,7 +513,7 @@ on_web_description_label_link_activated_cb (GtkLabel            *label,
 {
   GDBusConnection *connection = g_application_get_dbus_connection (g_application_get_default ());
 
-  gcal_utils_launch_online_accounts_panel (connection, NULL, NULL);
+  gcal_utils_launch_gnome_settings (connection, "online-accounts", NULL);
 }