#307 Add SetSockFd() method to PGstObject

[Victor Jaquez]

As in GStreamer 1.0 the udpsrc neither the udpsink uses integers
for socket descriptors but GSockets instances, this patch adds the
method SetSockFd() to configure any element that accepts the
properties "sockfd"/"closefd" in GStreamer 0.10 or
"socket"/"close-socket" in GStreamer 1.0

It goes on top of https://sourceforge.net/p/opalvoip/patches/306/