This menu allows you to have separate configurations for each application (browser, file manager, etc) opened in a different activity than the default.
In a nutshell: New activity = applications with different configurations from one activity to another.
This is not a plasmoid but a script. To use it, just download the package and insert the script into the plasma bar(Watch the video to get a better idea).
I'm not a developer and that's all I've managed to do and I have no idea how to make it a real plasmoid, so don't expect me to improve it(I don't say this badly, but I'm not able to do it.).
I am publishing it in this section in order to give visibility to the idea behind it.