Fetch theme editor items from ThemeDB

[KoBeWi]

Together with #84760 fixes #82402

[YuriSizov]

Needs a rebase, but otherwise looks good.

Do you think we leave any performance on the table by switching from a method pointer to get_theme_item_list? I know this is needed for ThemeDB anyway, so it's fine. But I'm not well versed in C++ so curious what your take is here.

[YuriSizov]

Actually, as I was testing #84760, I realized there is something wrong will all 3 PRs. While we can use get_class_own_items in the docs (and in fact we should), for the theme editor, the override list, and the argument autocompletion we need to include inherited items as well.

This requires a new get_class_items method, which returns not just own items. Plus we need a method to get such items filtered based on the data type, as outlined above. I think one of these PRs need to do all this groundwork, and then others can be built on top of it.

Let me know if you need any help with any of that.

PS. If you need an example of this being a problem, check CheckBox for instance.

[KoBeWi]

Updated. I added yet another argument to get_class_items(), which can be used to filter by type. If I were to add a new method, it would be pretty much the same, so I wanted to avoid code duplication.

[akien-mga]

Code changes look fine to me.

[akien-mga]

Thanks!