Implement name "renew" in Qt

Fix https://github.com/namecoin/namecoin/issues/131:  This patch implements a "Renew name" UI operation accessible both as new button on the "Manage names" page as well as via the name list context menu.  The operation simply does a name_update with the name's current value, and is thus a "shortcut" to using the "Configure Name..." dialog without changing anything.