Add subscriptions on the fly

[sionhughes]

Hi!

I'm a new user, having moved away from MQTT.fx which has become unusable at times (won't moan here).

I really like MQTT-Explorer, however one thing I'm missing is the ability to add subscriptions on the fly. Use case is that I can start and connect with set topics, then as I investigate an issue I can add other subscriptions and see other data. I know I could just use a subscription to '#' but in doing this I'd be getting way more data than I ever need. To give background, the project I'm working on has MQTT as a way of sharing data on an embedded platform with small measurements passed around alongside large time series data.