Add graphics option to toggle mouse cursor

Adds an option to show or hide the system mouse cursor over the game window.

desktop_version/src/Graphics.cpp

@@ -124,6 +124,7 @@ Graphics::Graphics()
     warprect = SDL_Rect();
 
     translucentroomname = false;
+    showmousecursor = true;
 }
 
 Graphics::~Graphics()