Codechange: Make all NWidgetPart arrays constexpr.

This ensures that the arrays are not created at runtime and prevents using non-constexpr values.

Codechange: Replace mishmash of types for widget index with WidgetID.

Indices were stored as int, but often passed around as uint/uint8_t and casts.

Now they should all use WidgetID.

Fix ab1a4c6c: Crash if the "No Music" set is loaded because there is no current set_index. (#11511)

Add a helper function to get the current set_index which which checks it is in range.

Fix ab1a4c6c: Changing baseset/playlist while shuffle is on does not change song. (#11510)

The change-playlist function relied on toggling shuffle to restart playing which is no longer the case, so always handle it when changing playlist instead.

Change: Don't restart playback when toggling playlist shuffle.

Instead update the selected playlist entry for the current song.

Codechange: Remove FONT_HEIGHT_... macros. (#11481)

These make it look like we're dealing with a constant, but actually each is a call to `GetCharacterHeight(...)`.

Codechange: Add __FILE__/__LINE__ of WindowDesc to WindowDesc.

This is to allow unit-tests to produce more useful output.

Codechange: Use begin/end of nwidget parts of begin/length.

This simplifies processing nwidget parts as, unlike the remaining length, the pointer to the end of the list never changes. This is the same principle as we use(d) for tracking end instead of length for C-style strings.

And this removes 160~ instances of the lengthof() macro.

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: Make all dropdown lists extend width if necessary.

This removes the auto_width parameter from ShowDropDown(At).

Fix: Don't (briefly) switch from title-only playlist on menu screen.

Switching playlist while on the menu screen now only changes the playlist
that will be used when out of the menu screen.

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: Use Rect helpers for widget drawing.

This replaces repetitive and sometimes unwieldy use of constants.

Codechange: Switch DropDownList to directly use std::vector, thus making AutoDeleteSmallVector obsolete.

DropDownListItem are strongly managed using std::unique_ptr to ensure leak-free handling. Appropriate use
of move-semantics make intent a lot clearer than parameter comments and allows the compiler to generate
copy-free code for most situations.

Change: Make volume slider widget capture mouse when clicked.

This makes the slider continue to respond even when mouse cursor
is no longer over the widget.

Fix 6298b96: Playlist window not drawing playlist

Copy-paste error in change to remove C++11 usage...

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.

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

Preparation for later extending the info passed to music drivers.

Fix: Clipping issues with volume sliders in Music GUI

Tested with various languages and GUI font sizes, should look good everywhere.

Also clamps near-end values to minimum and maximum so 0 and 127 are possible to achieve even on small GUI sizes (like the default.)