Allow advanced MM patching of Subcategory definitions

[Rodg88]

Having Subcategories in top level/root confignodes means MM can't use more advanced checks for patching, such as & for "only create if doesn't already exist".

For eg, this fails:

&ORGANIZERSUBCATEGORY[interstages]
{
  name = interstages
  Label = Interstages
  Priority = 12
  CategoryPriority = 30
}

with the error Error - create command (&) is not valid on a root node

The current solution of just being able to handle duplicate subcategories may be good enough to not have to do anything about this, but having ORGANIZERSUBCATEGORY being a subnode of a VABO root node may be a better solution.

[ChrisAdderley]

This is kinda unlikely to happen because it will require everyone to rewrite their patches.