Codechange: Add const versions of GetItem/GetGroup, and sprinkle liberally.

Non-const version of GetItem is not needed.

Codechange: Use std::list instead of C-linked list for ini files.

This removes self-management of memory with new/delete and allows simpler iteration.

Codechange: Remove create parameter from IniLoadFile::GetGroup.

GetGroup now only returns nullptr if the group does not exist.
Use GetOrCreateGroup to create a group.

This avoids creating groups while reading ini files.

Codechange: Split GetGroup into GetGroup/GetOrCreateGroup.

This follows the pattern used for GetItem/GetOrCreateItem, and allows use
of references where we know the group must exist.

Codechange: Add CreateGroup/CreateItem methods for ini files.

This abstracts the internals a bit.

Codechange: Split GetItem with GetOrCreateItem. (#10952)

`IniGroup::GetItem()` returns nullptr if the item does not exist, but does not if the create parameter is set to true. Resolve CodeQL warnings with `GetOrCreateItem()` which returns a reference to the item instead.

Change: include fmt.h C++ headers in stdafx.h

This to prevent compilation issues between runs with and without precompiled
headers. Also remove the headers from the rest of the code base as they are
not needed there anymore, although they do relatively little harm.

Change: move sensitive information to secrets.cfg and private information to private.cfg

We often ask people for their openttd.cfg, which now includes their
passwords, usernames, etc. It is easy for people to overlook this,
unwillingly sharing information they shouldn't.

By splitting this information over either private.cfg or secrets.cfg,
we make it more obvious they shouldn't be sharing those files, and
hint to what is inside them.

(svn r14164) -Codechange: simplify and unify the addition of ini items with value when not loading an ini file.
-Fix: wrong insertion management causing leaks.