The menus are growing too large. This patches move some categories under sub-menus, and shuffle some entries around. We already had sub-categories split by separators. This change now goes a step further and embrace 3-level menus. Inspired by the "Simpler Add Menu" add-on by Quackers (waiting to hear back to know Quackers real name). Inspired by the "Simpler Add Menu" add-on by Alfonso Martinez II. Differential Revision: https://developer.blender.org/D16993