gamecore/scenes: add a wrapper around Scene changing

gamecore/modules/scenes.lua

@@ -40,7 +40,7 @@ function Scene:new()
 end
 
 function Scene:register()
-  core.scenemanager.currentScene = self
+  core.scenemanager:setScene(self)
 end
 
 function Scene:update(dt)

gamecore/scenemanager.lua

@@ -30,6 +30,10 @@ function SceneManager:new(controller)
   self.currentScene = nil
 end
 
+function SceneManager:setScene(scene)
+  self.currentScene = scene
+end
+
 function SceneManager:update(dt)
   if (self.currentScene ~= nil) then
     local keys = self.controller.input.keys