Expands compositor definition keys into {type}.{group}
        keys. For instance a compositor operation returning a group
        string 'Image' of element type RGB expands to 'RGB.Image'.

def expand_compositor_keys(cls, spec):
        """
        Expands compositor definition keys into {type}.{group}
        keys. For instance a compositor operation returning a group
        string 'Image' of element type RGB expands to 'RGB.Image'.
        """
        expanded_spec={}
        applied_keys = []
        compositor_defs = {el.group:el.output_type.__name__
                           for el in Compositor.definitions}
        for key, val in spec.items():
            if key not in compositor_defs:
                expanded_spec[key] = val
            else:
                # Send id to Overlays
                applied_keys = ['Overlay']
                type_name = compositor_defs[key]
                expanded_spec[str(type_name+'.'+key)] = val
        return expanded_spec, applied_keys