Make required virtual methods abstract

[CedNaru]

So far we have ignored the "isRequired" field for methods. This property is always used alongside virtual methods and indicate that if you inherit its class with a script, it's mandatory to override this virtual methods (while others are just optional). In such case, I made the method (and the class) abstract instead of giving it a dummy implementation.

A side effect is that several classes in the Godot API inherit those now "abstract" classes, so I had to create dummy implementation for those abstract methods.