#147 Refound the plugin system

[Stéphane Aulery]

Refound the plugin system to make it complete and simple

Some examples:

• Wordpress
• Kanbord
• Adminer
• GLPI

Some papers:

• https://www.martinfowler.com/bliki/MonolithFirst.html
• https://stackoverflow.com/questions/2768104/how-to-create-a-flexible-plug-in-architecture
• https://www.martinfowler.com/eaaCatalog/plugin.html
• https://martinfowler.com/articles/injection.html
• https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/LoadingCode/Concepts/Plugins.html
• https://fman.io/docs/plugins-architecture

TODO:

• Only one entry point for dialogs and index pages
• Don't start session before session configuration (in WEBSERVERCONFIG.php)