TTS: Refactoring

* Delete multiple empty rows
 * Make getVolume non-virtual and leave just the implementation
    in base class
 * Resolve warning about signed / unsigned comparison in
    gui-manager
 * Clear availableVoices when updating voices on linux
 * By default set language to transMan language on windows
    (if the transMan is available)
 * Remove freeVoices method from Windows ttsMan, it isn't needed
    anymore

backends/text-to-speech/linux/linux-text-to-speech.h

@@ -59,7 +59,6 @@ public:
 	virtual void setPitch(int pitch);
 
 	virtual void setVolume(unsigned volume);
-	virtual int getVolume();
 
 	virtual void setLanguage(Common::String language);