add diagrams.net xml export

[SteenHBasilicom]

• add diagrams.net xml export capabilities
• split yEd and diagrams from main command
• update Readme

[AlexanderHeidrich]

So the diagrams.net output is not working. Especially for the getFieldtype methods.

In DiagramsPimcoreDefinitionsRepository.php line 145:
                                                                                                                                
Attempted to call an undefined method named "getFieldType" of class "Pimcore\Model\DataObject\ClassDefinition\Layout\Panel". 

I guess we need recursive calls here?