Provide a `Skeleton.readonly()` function

[phorward]

Shortcut, to immediately obtain a readonly skeleton.

Avoid things like these...

    def baseSkel(self):
        # Make all bones readOnly!
        # FIXME: There should be a skel.readonly() function soon...
        skel = super().baseSkel().clone()

        for bone in skel.values():
            bone.readOnly = True

        return skel