Add: store headers for most savegame chunks

When a header is added, the chunk changes from CH_ARRAY type to
CH_TABLE type.

Codechange: move network-related settings out of settings.ini

This to prepare the code to split up network-related settings
into private / secrets / generic.

Fix #9386: compilers failing to compile with LTO by using variants instead of new + unique_ptr

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.

Codechange: use C-style strings instread of std::string in the SettingDesc constructor

This as using std::string causes much more variables to be tracked, potentially causing problemes for certain compilers in certain situations

Codechange: merge guiflags and flags in settings .ini files

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.

Codechange: rename SettingGuiFlag to SettingFlag (#9332)

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.

Codechange: use setting name instead of index for CmdChange(Company)Setting (#9306)

This is mostly done as there are now constraints on settings.ini you might not
expected. For example, conditional settings always have to come last, as otherwise
they would influence the index.