Introduction of a specialized InvisibilityChanger for the notebook. It
allows to handle more elegantly the issues of the notebook during
page execution: modifiers hiding the active tab that must now
active its sibling.
Process_notebook only handle the processing of the page while
NotebookInvisibilityChanger only handle the behavior of the notebook
during the page execution.
- readable code (more readable, at least)
- correct handle of autofocus
- rendering of the whole notebook in one call to qweb's render
- one invisibility changer bound by header/content
- no custom css, only legacy bootstrap