MQTT data streamer only subscribes to single topic

[avanover]

Problem description

The mosquito-based MQTT data streamer plugin allows the user to list topics with a wildcard, but then requires the use to select a specific topic. If the user wants to subscribe to multiple topics, this is not possible.

Steps to reproduce (important)

• Duplicated on macOS
• Have a test program publish to two MQTT topics (named 'messages/msg1' and 'messages/msg2')
• Start PJ and connect to MQTT
• Use the default value of '#' for the Topics filter and click "Connect"
• 'messages/msg1' and 'messages/msg2' should be displayed, but the user has no way to select both and must only select one

[avanover]

Created pull request #745 to address this.