Introducing `AdminPagePreBuild` delegate

This delegate is needed in some cases in order for extensions to do things before the content pages (JSON/XML/...) are instantiated. Using `AdminPagePreGenerate` was not working in some cases because the `view()` method (where most of the code for content page goes) is called by the `build()` method called in `__buildPage()`.

I've marked it as @since 2.6.0 but please tell me to change it if that's not ok. (Is there a 2.5.3 planned?)