Added a method to check which plugins support invitations.

api/net/sf/briar/api/plugins/Plugin.java

@@ -33,4 +33,7 @@ public interface Plugin {
 	 * callback.
 	 */
 	void poll();
+
+	/** Returns true if the plugin supports exchanging invitations. */
+	boolean supportsInvitations();
 }

components/net/sf/briar/plugins/bluetooth/BluetoothPlugin.java

@@ -264,6 +264,10 @@ class BluetoothPlugin extends AbstractPlugin implements StreamPlugin {
 		return url == null ? null : connect(c, url);
 	}
 
+	public boolean supportsInvitations() {
+		return true;
+	}
+
 	public StreamTransportConnection sendInvitation(int code, long timeout) {
 		return createInvitationConnection(code, timeout);
 	}

components/net/sf/briar/plugins/file/RemovableDrivePlugin.java

@@ -59,6 +59,10 @@ implements RemovableDriveMonitor.Callback {
 		throw new UnsupportedOperationException();
 	}
 
+	public boolean supportsInvitations() {
+		return true;
+	}
+
 	@Override
 	protected File chooseOutputDirectory() {
 		try {

components/net/sf/briar/plugins/socket/SimpleSocketPlugin.java

@@ -97,13 +97,15 @@ class SimpleSocketPlugin extends SocketPlugin {
 		callback.setLocalProperties(p);
 	}
 
+	public boolean supportsInvitations() {
+		return false;
+	}
+
 	public StreamTransportConnection sendInvitation(int code, long timeout) {
-		// FIXME
+		throw new UnsupportedOperationException();
-		return null;
 	}
 
 	public StreamTransportConnection acceptInvitation(int code, long timeout) {
-		// FIXME
+		throw new UnsupportedOperationException();
-		return null;
 	}
 }