Introduce a MessageContext class for more flexibility

This change will allow to pass message dependencies from the client
validators to the ValidationManager.

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

@@ -0,0 +1,28 @@
+package org.briarproject.api.clients;
+
+import org.briarproject.api.data.BdfDictionary;
+import org.briarproject.api.sync.BaseMessageContext;
+import org.briarproject.api.sync.MessageId;
+
+import java.util.Collection;
+
+public class BdfMessageContext extends BaseMessageContext {
+
+	private final BdfDictionary dictionary;
+
+	public BdfMessageContext(BdfDictionary dictionary,
+			Collection<MessageId> dependencies) {
+
+		super(dependencies);
+		this.dictionary = dictionary;
+	}
+
+	public BdfMessageContext(BdfDictionary dictionary) {
+		this(dictionary, null);
+	}
+
+	public BdfDictionary getDictionary() {
+		return dictionary;
+	}
+
+}