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

Non-const version of GetItem is not needed.

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.

Fix: Truncated music-set song names cause warning log.

The music-set does not need to be selected for this to occur.

Resolved by using std::string instead of fixed buffer for song names,
which avoids manual string copying and removes the length limit.

Change: Modernise music control logic implementation (#6839)

Rewrite of almost the entire music control logic to a more modern style, hopefully also easier to understand. The old playlist handling made it look like arcane magic, which it doesn't have to be.

- Playlists are now stored in std::vector of objects instead of arrays of bytes with magic sentinel values, that need to be rotated around all the time. Position in playlist is stored as a simple index.
- The theme song is now reserved for the title screen, it doesn't play on any of the standard playlists, but is still available for use on custom playlists.
- When the player enters/leaves the game from the main menu, the music always restarts.
- Playback state (playing or not) is kept even if music becomes unavailable due to an empty playlist (or an empty music set), so it can restart immediately if music becomes available again.
- The shuffle algorithm was changed to a standard Fisher-Yates.
- Possibly better behavior when editing a custom playlist while it's playing.
- Custom playlists should be compatible.
- Framework for supporting custom playlists with songs from multiple music sets.

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

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

Change #6684: Cutting point overrides for music base sets

This improves bad looping of title screen song from Windows TTD, and fixes
a long silence at the end of "Can't get there from here" from Windows TTD.

Feature: Decoder for DOS version music

This is based on reverse-engineering the TTD DOS driver for General MIDI music.

Codechange: Pass a MusicSongInfo struct instead of bare filename to music drivers.

Preparation for later extending the info passed to music drivers.