added a structure in GxsNetService to record who sends info about groups, so as to dynamically compute group popularity. The GUI needs to be improved to update regularly so as to reflect current values for popularity. For now only subscribing/unsubscribing and new messages trigger the update

git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@7587 b45a01b8-16f6-495d-af2f-9b41ad6348cc