Renames menu_subcategory to subcategory - first cut

Subcategory is always a part of a menu.
Double qualification in name is unnecessary.
Renames header clauses to match the class names
Changes file name references from menu_subcategory to category - second cut
Renames menu_subcategory class to category class - third cut

Maintains compatibility with usage by creating empty shim class for menu_category
Changes usages of menu_subcategory to subcategory - fourth cut
Removes menu_category compatibility shims - fifth and final cut