Prepare for new Forum Sharing Client

Methods for creating, adding and removing forums have been moved to the
`ForumManager`. In order to still handle removing forums properly, a
`RemoveForumHook` has been introduced.

Methods for sharing forums with all current and future contacts have
been removed along with the localGroup where this information was saved.

The `ShareForumActivity` now has the proper label.

The `SessionId` and the `ProtocolEngine` have been moved to the
`clients` package.

This addresses part of #322 and part of what has been discussed in #320.

briar-api/src/org/briarproject/api/clients/SessionId.java

@@ -0,0 +1,19 @@
+package org.briarproject.api.clients;
+
+import org.briarproject.api.sync.MessageId;
+
+/**
+ * Type-safe wrapper for a byte array
+ * that uniquely identifies a protocol session.
+ */
+public class SessionId extends MessageId {
+
+	public SessionId(byte[] id) {
+		super(id);
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		return o instanceof SessionId && super.equals(o);
+	}
+}