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/includes/templates/admin/edit_elements.tpl

@@ -114,8 +114,8 @@
 			{/foreach}
 			</table>
 			{if $element.data.delete_name}
-			<input type="hidden" value="" name="{$element.data.delete_name}">
-			<input type="hidden" value="" name="{$element.data.delete_name}_flag">
+			<input type="hidden" value="" id="{$element.data.delete_name}" name="{$element.data.delete_name}">
+			<input type="hidden" value="" id="{$element.data.delete_name}_flag" name="{$element.data.delete_name}_flag">
 			<input type="hidden" name="remove_name[]" value="{$element.data.delete_name}">
 			{/if}
 			{if $element.data.enable_name}