Project group and some optimization for faster retrieval of workitems.

[WH-Yoshi]

I do this PR to add my personal use case (project group) as it can be useful. It's only here as a workitem retriever for now but can be largely modified.

It's implemented so it's not more complicated to use than a project. I believe that my code is clean, but any review is very welcomed as I'm not an expert.