TTS: Add method to get the default voice

This returns by default the first voice, but can be reimplemented
in derived classes to return the system default voice.

common/text-to-speech.h

@@ -293,6 +293,11 @@ public:
 	 */
 	Array<int> getVoiceIndicesByGender (TTSVoice::Gender gender);
 
+	/**
+	 * returns the index for the default voice.
+	 */
+	virtual int getDefaultVoice() { return 0; }
+
 	/**
 	 * Pushes the current state of the TTS
 	 */