Add: Website button to Game Options window for basesets.

This links to the optional "url" item in the baseset's [metadata] section.

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: Pass ini file by reference and prefer automatic storage.

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

Codechange: automatic adding of _t to (u)int types, and WChar to char32_t

for i in `find src -type f|grep -v 3rdparty/fmt|grep -v 3rdparty/catch2|grep -v 3rdparty/opengl|grep -v stdafx.h`; do sed 's/uint16& /uint16 \&/g;s/int8\([ >*),;[]\)/int8_t\1/g;s/int16\([ >*),;[]\)/int16_t\1/g;s/int32\([ >*),;[]\)/int32_t\1/g;s/int64\([ >*),;[]\)/int64_t\1/g;s/ uint32(/ uint32_t(/g;s/_uint8_t/_uint8/;s/Uint8_t/Uint8/;s/ft_int64_t/ft_int64/g;s/uint64$/uint64_t/;s/WChar/char32_t/g;s/char32_t char32_t/char32_t WChar/' -i $i; done

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.

Remove: ENABLE_NETWORK switch

This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.

Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.

A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.

With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.

All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.

Fix: Don't complain if CAT music files are missing entirely

Just complain if an index into a CAT file that exists is invalid.