Add volume slider for title/background music (#1130)

* add volume slider

* add translations

* stop music when checkbox unchecked

* remove GUI build command args

* combine functions

* add accidentaly removed copyright and licencing information
(thanks QT Designer)

src/qt_gui/background_music_player.h

@@ -16,6 +16,7 @@ public:
         return instance;
     }
 
+    void setVolume(int volume);
     void playMusic(const QString& snd0path);
     void stopMusic();
 
@@ -25,4 +26,4 @@ private:
     QMediaPlayer* m_mediaPlayer;
     QAudioOutput* m_audioOutput;
     QUrl m_currentMusic;
-};
+};