Support abstract classes

[chippmann]

This adds support for member registration in abstract classes.

Now one can define abstract classes which inherit from a godot type and register default members in it.
Upon registration of inheriting classes, the inheriting class registers the member of the abstract class as well.

To godot, the abstract class does not exist as it's not registered. But on the jvm side, it exists and all calls to the default members are properly propagated through normal inheritance.

Example:

// register class annotation is optional for abstract classes
abstract class AbstractClassInheritanceParent: Node() {

    @Export
    @RegisterProperty
    var registeredExportedPropertyInAbstractClass = false

    @RegisterSignal
    val signalInAbstractClass by signal<String>("blubb")

    @RegisterFunction
    fun functionInAbstractClassWithDefaultImplementation() {
        // some implementation
    }

    @RegisterFunction
    abstract fun abstractFunction()
}

@RegisterClass
class AbstractClassInheritanceChild: AbstractClassInheritanceParent() {
    @RegisterFunction
    override fun abstractFunction() {
        // some implementation
    }
}

Limitations:
Currently a user must explicitly register any member he overrides even if that member is marked as registered in the abstract class.
This could be changed though in the future, but personally i would opt against it, to force users to be explicit about what is exported and what not.

Note to self: If this PR is accepted, it needs to be cherry picked to the 4.0 branches