Declarative submenus

Created by: portnov

Addressed problem description

• Some categories became too large
• Manual manipulation with categories, like in juggle_and_join, is too long and will get more and more hard to maintain.

Solution description

Add a possibility to make submenus declaratively, from index.md:

• Sub-category must be named as Category @ Submenu title in index.md
• Submenu can be inserted into category by @ Submenu title construct
• Submenus have to be registered in nodeview_space_menu.py / classes list
• Sub-categories do not appear as separate top-level menus, they appear as submenus in containing category as submenus instead
• In toolbar, they do not appear as separate categories, they are included in contained categories.

Preflight checklist

Put an x letter in each brackets when you're done this item:

• Code changes complete.
• Code documentation complete.
• Documentation for users complete (or not required, if user never sees these changes).
• Manual testing done.
• Unit-tests implemented.
• Ready for merge.