Pad: allow building a Pad with an automatically generated name

For convenience, the Pad builder checks a name is provided when a wildcard-named template is used. For GhostPads, the builder tries to assign the name of the target Pad making sure the provided name conforms to the PadTemplate.

This commit adds a function to optionally keep the gst::Object automatically generated unique Pad name (such as ghostpad4) and reorganises name handling so it is processed when build is invoked.