Break out the game demo into multiple source and include files, add a bunch of missing assets, conform to the new subsystems interface from libakgl

assets/sprites/menupointer.json

@@ -0,0 +1,16 @@
+{
+    "spritesheet": {
+	"filename": "../menupointer.png",
+	"frame_width": 50,
+	"frame_height": 49
+    },
+    "name": "menupointer",
+    "width": 50,
+    "height": 49,
+    "speed": 0,
+    "loop": false,
+    "loopReverse": false,
+    "frames": [
+	0
+    ]
+}