SCI: Text to Speech (TTS) functionality for SCI16 (SCI0 - SCI1.1) games

This is based on the work done for GSoC by @taylorzhancher in PR #3256

Highlights:
- TTS has been hooked globally onto places where text is shown
- TTS is currently performed for textboxes and button texts
- TTS stops when text windows are disposed (which enhances the in-game
  experience)
- No game-specific logic has been added
- This hasn't been extensively tested with all SCI16 games yet
- There will be cases that are not handled properly yet
- The TTS functions have been grouped under a SciTTS class

engines/sci/detection_options.h

@@ -185,6 +185,18 @@ const ADExtraGuiOptionsMap optionsList[] = {
 		}
 	},
 
+#ifdef USE_TTS
+ 	{
+ 		GAMEOPTION_TTS,
+ 		{
+ 			_s("Enable Text to Speech"),
+ 			_s("Use TTS to read the descriptions (if TTS is available)"),
+ 			"tts_enabled",
+ 			false
+ 		}
+ 	},
+#endif
+
 	AD_EXTRA_GUI_OPTIONS_TERMINATOR
 };