Fix unmounting assets exiting to menu resulting in silence

If you exited to the menu normally (i.e. got on a code path that went
through Game::quittomenu()), the menu music wouldn't play. This is
because FILESYSTEM_unmountassets() was put after music.play(6). So the
game would play the custom level's track 6, and then unmount it, which
meant it could no longer play track 6, but there's nothing telling the
game to play track 6 again. So I just changed the frame ordering around.

I also added a comment to make sure anyone reading the code is aware of
the frame order dependency.

desktop_version/src/Game.cpp

@@ -7631,6 +7631,7 @@ void Game::quittomenu()
 {
     gamestate = TITLEMODE;
     graphics.fademode = 4;
+    FILESYSTEM_unmountassets(); // should be before music.play(6)
     music.play(6);
     graphics.backgrounddrawn = false;
     map.tdrawback = true;
@@ -7669,7 +7670,6 @@ void Game::quittomenu()
         createmenu(Menu::mainmenu);
     }
     script.hardreset();
-    FILESYSTEM_unmountassets();
 }
 
 void Game::returntolab()