As the title states, it would probably be an intuitive feature to have a “+” button within categories (maybe on hover it appears) that allows you to quickly add a project to that category. It’s a little clunky to add it from the bottom of the left sidebar IMO – and then to have to find the project as well.
Edit: On right-click is fine for now but not as intuitive either since right-click features are more hidden in comparison.
This is the classic tradeoff between complexity (visually the plus adds clutter) and power use. We feel the balance as it is probably what we want, the power user approach is to indeed use the right-click option to directly add a new project to the category.
Right, the problem is that in this case I would argue the complexity is still there. The only difference is that the complexity is hidden in menus. Plus, I’m used to most power features being implemented for things that people do less often, not more often. I think adding projects into categories is a pretty frequent action.
But hey, the feature is there, and it works, that’s what matters most. 
Edit: Also, I realize you could probably throw a setting like that within the “…” UI menu. 
I just wanted to +1 this. I constantly click on the “…” buttons on the category when I want to add a project.