First draft of the NewGameMenu class:
* creation of an empty LineEdit class
* adding attributs and methods to store and restore last state in MenuManager
* moving all initialization code of each Menu in a new "init" virtual method
First draft of the ManageGamesMenu :
* creation of the empty ManageGamesMenu class
* creation of the empty ScrollArea class
* creation of the empty Slider class
NOTE : need to solve the input issues to continue ( input cames by three each loop and the event is too quickly transmitted in menu tree)