Extract the layout proc into a method, and write documentation explaining what the proc does in various cases.

Break up DependencyModule's dual function of providing a "depend_on" DSL and "included" block DSL into separate modules. But, unify both approaches under AS::Concern.

Deprecate template, session, assigns, and layout accessors on response object. Instead access them through the controller instance. This mainly affects functional test assertions.

  ActionView::Template is now completely independent from template
  storage, which allows different back ends such as the database.
  ActionView::Template's only responsibility is to take in the
  template source (passed in from ActionView::Path), compile it,
  and render it.

  * Decouple from ActionController and ActionMailer
  * Bring back localization support.
  * Prepare to decouple templates from the filesystem.
  * Prepare to decouple localization from ActionView
  * Fix ActionMailer to take advantage of ActionView::Path