SCI: Improve audio volume & settings sync code

This patch includes enhancements to the ScummVM integration with
SCI engine, with particular focus on SCI32 support.

1. Fixes audio volumes syncing erroneously to ScummVM in games
   that modify the audio volume without user action (e.g. SCI1.1
   talkies that reduce music volume during speech playback). Now,
   volumes will only be synchronised when the user interacts with
   the game's audio settings. This mechanism works by looking for
   a known volume control object in the stack, and only syncing
   when the control object is present. (Ports and planes were
   researched and found unreliable.)

2. Fixes audio syncing in SCI32 games that do not set game
   volumes through kDoSoundMasterVolume/kDoAudioVolume, like GK1,
   GK2, Phant1, and Torin.

3. Fixes speech/subtitles syncing in SCI32 games that do not use
   global 90, like LSL6hires.

4. Fixes in-game volume controls in SCI32 games reflecting
   outdated audio volumes when a change is made during the game
   from the ScummVM launcher.

5. Fixes SCI32 games that would restore volumes from save games
   or reset volumes on startup, which caused game volumes to be
   out-of-sync with ScummVM when started.

6. ScummVM integration code for audio sync has been abstracted
   into a new GuestAdditions class. This keeps the ScummVM-
   specific code all in one place, with only small hooks into the
   engine code. ScummVM integrated save/load code should probably
   also go here in the future.

Fixes Trac#9700.

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