Added panels and menu
* Added class Panel
* Added class MenuPanel
* Removed classes Renderable and Updatable
* Renamed class Game to Application, plus some changes :
    * added a panel system
    * added methods addPanel(Panel*) and removePanel(Panel*)
    * modified methods update() and render()
    * added destructor
* Modified class Background :
    * added method setSprite(sf::Sprite)
* Modified class GUIElement
    * removed inheritance
* Modified main :
    * changed game to application
    * now adds the menu as a panel
* Modified CMakeLists.txt :
    * Added and removed some files from SOURCES