This reduced the load on compilers, as currently for example MacOS
doesn't like the huge settings-tables.
Additionally, nobody can find settings, as the list is massive and
unordered. By splitting it, it becomes a little bit more sensible.
With std::variant all memory can be figured out at compile time, so the compiler needs to keep track of fewer elements. It also saves out a unique_ptr and its memory management, over a slight impact for resolving a setting.
It was rather confusing which one was for what, especially as some
SaveLoad flags were settings-only. Clean up this mess a bit by
having only Setting flags.
It is a lovely organicly grown enum, where it started off with
GUI-only flags, and after that a few flags got added that can be
considered GUI-only (the GUI disables/enables based on them), to
only have flags added that has nothing to do with the GUI.
So be less confusing, and rename them to what they do.
Additionally, I took this opportunity to rename 0ISDISABLED to
reflect what it really does.
