Do not initialize abstract classes

[LucasDemea]

I found myself in a situation where I needed to declare an abstract class, and it was automatically picked and initialized by Engine\Initialize, resulting in an error. We can fix this by first checking if the class we are going to initialize isn't an abstract one.

[Mte90]

Thanks for spotting it!

I think that the initializer needs some rework to simplify it but if I add some explanation to release the plugin with the autoload dumped by composer there should not be issues for end-users.