Refactor paged dialogs

* Make each page an object with its own class. The new BookletLayout
  takes PageLayout objects instead of plain objects.

* Add continuous mode for stack layouts.

* Synchronize continuous stack layouts with outlines in booklet layouts.

* Simplify names of other layouts.

Transposed from I51e74b322ae in the VisualEditor repository.

Change-Id: Id0fb0aab1999d0ba93fcfd192ac33e97e97e93e0
Merge "Refactor paged dialogs"