Suggestion: Allow selection of QOS level

[MrAureliusR]

In the Session box, it would be nice if the user could choose the QoS level (0, 1 or 2).
It would also be awesome if the user could supply a last will message, as well as optionally mark a message as a retained message. When a message received is marked as a retained message, it would be nice to possibly change the background of this message.

I suppose this is two suggestions in one issue :3

[MayaPosch]

Initial support for retain & QoS has been added to the source code. It's still fairly buggy, however, and needs more debugging both in MQTTCute and NymphMQTT.

[MayaPosch]

Fixed an issue in NymphMQTT that made things not work. Feel free to test QoS and retain in MQTTCute now. It was working on my end at least :)