I've searched for existing issues

• Yes, I have.

Description

While developing projects, many times I need to send the same (few) messages to test my new functionality. I'm sure I'm not the only one with this workflow. What I have in mind is a small area (hidden by default) where I can store a number of messages that I can send to the selected topic simply by selecting them (or selecting them and click Send). An even better version of this would be where not only some pre-made messages can be selected, but message/topic pairs.
Currently the process is that I have these messages saved in an external file, then I copy/paste them in MQTT Viewer - tedious/error prone.
I realize the command history is something similar, but it's not very practical (for my use case) as the previous messages are all there and so it is difficult to pick which one I want (as they keep moving).

Inspiration / Screenshots

Similar functionality I have used in the past in MQTTMultimeter, which is otherwise not very usable to me:

The Untitled part is a mystery to me, please ignore it :), Anyway, it's a list of topic/payload(message) combos.

I have also used a similar concept (not in the MQTT realm) in ScriptCommunicator. Here you also have predefined messages (called sequences here) to send, in this case, over a serial port:

Just double click an entry to send it.

I think this would be a very useful feature for many. Let me know if you need further clarification, help with testing, etc.