config: Use common function for identifying 'general' subcats

...removing the code duplication and helping avoid mistakes.

Note that this temporarily reverts a small macos fix made in 08134b3f that involved the macos code no longer considering the 'advanced-misc' subcat as being a general one to try to address an issue displaying child nodes of that category in the prefs tree. The issue results from the facts that plugins under that category commonly target its general subcat, with the macos code not having been designed to properly support that. Work to be submitted in a later MR addresses that inadequacy of the macos code and thus solves the problem properly and without breaking the 'every-category-has-a-general-subcat' predicate that my upcoming rework of preference tree construction code somewhat relies upon.

(Split out from MR !307)