Swiften handles blocklist event notifications, but those were not passed to Sluift, so a Sluift client, when querying events, would not be aware if an XMPP server pushes block/unblock. This patch adds an extra event type to Sluift so that method for_each_event reports block/unblock notifications, in addition to a new API 'get_block_list' to retrieve the blocklist from the server. Test-information: Used Sluift client to retrieve blocklist and it is as expected. Also used client to retrieve all items after adding, removing and removing all items in the blocklist, and Sluift clients can now see these events. Change-Id: I0b76289ebd9e63505ff8a99cd9c0aa0e93af0c22