Get simple 3.3.16 support for php 8.0

[Abbe]

This project is too good to die yet, so lets keep it going.
I modified/replaced functions where it was neccessary to make the CMS PHP 8 compatiple.
Here is what I did:

Fixed bug in admin/changedata.php file line 52
Fixed bug in admin/changedata.php file line 60
Fixed bug in admin/upload.php file line 178

Modified/Replaced all functions using get_magic_quotes_gpc() in all relevant files.
PHP 8.0 no longer adds slashes to post variables so no need for stripslashes() the function get_magic_quotes_gpc() has been removed in PHP 8.0

Also added thumbnails to images when listing files in the Files tab.

Attaching the changed files, just unzip an drop in the GetSimpleCMS-3.3.16 folder to overwrite the previous versions of the changed files.

enjoy