[WIP] Allow Core Classes Overriding

Based on Yetiforce code, it will be great to have the ability to override core module classes. https://www.yetiforce.com/en/developer-documentation/engine/177-loader-and-autoloader-of-files.html Once applied, simply create a folder 'custom' at the root of your vtiger system and use the proper folder structure. This only allow overriding of core modules classes, it will be also useful to be able to override templates files, language files and other core files.