help: remodel core help text category headings

Currently there are two parallel sets of category markers within the core option set, the 'cat/subcat' one used by the GUI preference interfaces, and the 'cat-hints' one used in help text output.

The labels for the cat-hint model have had deprecation notices ever since v0.8.5, so ditching that system seems to have been wanted for a long time (almost 16 years). The cat-hint model is also a maintenance problem, considering the fixes I've had to make previously (711733c6).

This MR changes the core help text output to make use of the 'cat/subcat' model, ditching the legacy cat-hints model.

There are two variants of this to choose from as the final solution. (Currently Choice A is implemented, but Choice B just requires dropping a few lines).

• Choice A makes use of all subcat hints within the core option set.

  Screenshot comparing original with choice A (Original on the left)

  We get more headings with this compared to the original, but in places this seems like a bad thing.

• Choice B just uses the 'general' subcat entries (essentially root category headings only).

  Screenshot comparing original with choice B (Original on the left)

  This gives cleaner/simpler output, much closer to the original, with a few key differences:

  • Loosing the 'Subpictures' heading (which I think is fine given the section headings).
  • The 'Input' and 'Input access and codecs' sections get combined.
  • We loose the 'Hot keys' heading (those options now fall under 'Interface'). Please note that I'm intending to propose dropping most of the hotkey stuff from help text.

I'm leaning towards choice B personally.