Feature request: Select several folders as origin

[Korne127]

First of all, thank you very much for this nice add on, it really saved me.
However, I'd like to request a specific feature. I had a situation in which there was the default inbox folder that contained everything and several other folders which contained (exclusive) subsets. I then wanted to delete everything from the main inbox that didn't exist in these other folders.
However, per default the inbox folder was the one that was checked to keep the mail in. And while the possibility to right-click a different folder and select it as the origin exists, you couldn't do it with several folder (if you tried it, each selection would replace the last one, leading to e.g. folder A being selected over the inbox but the inbox selected over folder B).

Luckily, I could solve this problem by using the main inbox as default value and then clicking on the "always select last" option, but I can imagine other use cases in which this wouldn't work. Therefore, it would probably be a good idea to introduce such a feature (or even better, the possibility of adding origin priorities like a fallback list).