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

Non-const version of GetItem is not needed.

Codechange: Use range-for instead of nullptr-terminated list in settingsgen.

This simplifies iteration and avoids use of pointers.

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: Pass ini file by reference and prefer automatic storage.

This avoids new/delete operations, and (not) checking for nullptr.

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.

Fix: Make all settingsgen 'warnings' fatal. (#10766)

Compilation should stop If settingsgen fails to complete properly.

Codechange: validate that "max" value of settings fit in their storage

This is an easy mistake to make, so protect us against making such
mistakes, by validating it doesn't happen.

Remove: MorphOS / AmigaOS support

In 10 years there is no commit to change how MorphOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), MorphOS is no longer
support. There is an SDL2 port for MorphOS, but it is not maintained
by upstream SDL2, and nobody can currently test it out.

If anyone wants to re-add MorphOS, please do (revert this patch,
fix the problems, and create a Pull Request). If you need any help
doing so, let us know! It is not that we don't like MorphOS, it is
that we don't have anyone fixing the problems :(