Export and import default profile icon relation.
At export time, we export a few links in our persondata element that point to profile icons. The icons themselves link to the persondata element. In lieu of anything better, we're currently using 'related' for this. At import time, we look at links in the persondata element, and the first 'related' one we come across that is a profile icon, we set as the default. This conforms with what the list people were doing, so at least for this version of the spec we'll be doing it consistently with the others. Signed-off-by: Nigel McNie <email@example.com>