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
d4e638baac