[woob/browser/pages] multiple inheritance with AbstractPage

Use page pointed by AbstractPage, but also keep any other class declared with it.
ex: SomePage(SomeMixin, SomeOtherPage, AbstractPage)
This allows to use methods from other classes than the abstracted one.
This conserves order of classes' declaration

Plus add docstring for class AbstractPage.

Plus do not allow AbstractPage that is inheriting from an other.