Reorganise project menu structure

I found that it can be quite cumbersome to open stacks using the "Projects/Current" submenu, partly because it's prone to losing focus when mouse movement is not accurate.

Basically, three changes are introduced:
* The "Projects/Current" submenu is moved to a new top level menu called "Stacks"
* The "Projects/New" item is removed, since it's not being used (or is it?)
* The "Projects/Open" submenu is moved up one level to "Projects"

So if you think this is interesting for everyone, please feel free to try out and merge.