Only allow new introductions in START state

When the user attempts an introduction, instead of the introduction
message input field, an explanatory text will be shown and the
introduction can not be made until the last one has been finished.

briar-api/src/main/java/org/briarproject/briar/api/introduction/IntroductionManager.java

@@ -25,6 +25,8 @@ public interface IntroductionManager extends ConversationClient {
 	 */
 	int CLIENT_VERSION = 1;
 
+	boolean canIntroduce(Contact c1, Contact c2) throws DbException;
+
 	/**
 	 * Sends two initial introduction messages.
 	 */