provide a dict or kwargs to merge multiple values into skel (like dict.update())

More context, please!

Basically it's this method which should be added into SkeletonInstance:

    def update(self, *args, **kwargs) -> None:
        for key, value in dict(*args, *kwargs).items():
            self[key] = value    

So you can write:

skel.update(name="foo", sortindex=2)
# or
skel.update({"name": "foo", "sortindex": 2})

But this needs to be tested and we should think about if this should use setBoneValue and then we have to ensure this works for every bone, ...

I think this should make use of the __or__-operator as well, so one can write

skel |= {"name": "foo", "sortindex": 2}