reorganize modules

modules/background/background.lua

@@ -0,0 +1,22 @@
+Background = { }
+
+-- private variables
+local background
+
+-- public functions
+function Background.create()
+  background = UI.loadAndDisplay('/background/background.otui')
+end
+
+function Background.destroy()
+  background:destroy()
+  background = nil
+end
+
+function Background.hide()
+  background:hide()
+end
+
+function Background.show()
+  background:show()
+end