Update to make all class E_NOTICE safe, add page_content

- ALL classes are E_NOTICE safe as far as possible.
There might be some minor things left over which will be cleaned up in
further testing

- Added declare(strict_types=1); on all pages for trying to make all
calls strict

- Added page_content sub content to edit_page, with this some inner page
content with ACL can be set, eg for use with Ajax/JS calls with backend.
Also alias can be set so the control ajax pages can back reference to
the master page content setting. Currently only one back reference is
allowed

- Note that the PAGES array has no numeric indexes, but uses the cuid as
index

www/admin/error_test.php

@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 $DEBUG_ALL_OVERRIDE = 0; // set to 1 to debug on live/remote server locations
 $DEBUG_ALL = 1;