Refactor Model 2

[SeZuo]

Modify the Model class to store the following information:

• group name
• previous page
• current page
• next page
• list of all the menus of the groups in the "booklet" group
• a list of all the resources needed by the page.

It is best to load the current page before deferring the previous and next page load.

[SeZuo]

done