Links the two named pads of the source and destination elements.
Side effect is that if one of the pads has no parent, it becomes a child of the parent of the other element. If they have different parents,
the link fails. If
caps is not
null, makes sure that the caps of the link is a subset of
a Element containing the source pad.
the name of the Pad in source element or
the Element containing the destination pad.
the name of the Pad in destination element or
the Caps to filter the link, or