Add being able to mute the music by pressing N

This is for people who want to use their own soundtrack while playing
the game, but who don't want to mute the sound effects as well.

This feature was added to VCE, but it was added in the strangest way. It
was made an option in "game options" instead of being a keybind, and I
don't know why.

desktop_version/src/Game.cpp

@@ -119,6 +119,8 @@ void Game::init(void)
     infocus = true;
     paused = false;
     muted = false;
+    musicmuted = false;
+    musicmutebutton = 0;
     globalsound = 128;
     m_globalVol = 1.0f;
 

desktop_version/src/Game.h

@@ -192,6 +192,8 @@ public:
     bool infocus;
     bool muted;
     int mutebutton;
+    bool musicmuted;
+    int musicmutebutton;
 private:
     float m_globalVol;
 

desktop_version/src/KeyPoll.h

@@ -21,6 +21,7 @@ enum Kybrd
 	KEYBOARD_a = SDLK_a,
 	KEYBOARD_d = SDLK_d,
 	KEYBOARD_m = SDLK_m,
+	KEYBOARD_n = SDLK_n,
 
 	KEYBOARD_v = SDLK_v,
 	KEYBOARD_z = SDLK_z,

desktop_version/src/main.cpp

@@ -519,6 +519,16 @@ int main(int argc, char *argv[])
             game.mutebutton--;
         }
 
+        if (key.isDown(KEYBOARD_n) && game.musicmutebutton <= 0 && !inEditor)
+        {
+            game.musicmutebutton = 8;
+            game.musicmuted = !game.musicmuted;
+        }
+        if (game.musicmutebutton > 0)
+        {
+            game.musicmutebutton--;
+        }
+
         if (game.muted)
         {
             game.globalsound = 0;
@@ -529,8 +539,16 @@ int main(int argc, char *argv[])
         if (!game.muted && game.globalsound == 0)
         {
             game.globalsound = 1;
-            Mix_VolumeMusic(MIX_MAX_VOLUME) ;
             Mix_Volume(-1,MIX_MAX_VOLUME);
+
+            if (game.musicmuted)
+            {
+                Mix_VolumeMusic(0);
+            }
+            else
+            {
+                Mix_VolumeMusic(MIX_MAX_VOLUME);
+            }
         }
 
         if (key.resetWindow)