Introducing `AdminPagePreBuild` delegate

This delegate is needed in some cases in order for extensions to do things before the content pages (JSON/XML/...) are instanciated. 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`.
Merge pull request #2238 from DeuxHuitHuit/pre-build-delegate

Introduce a `AdminPagePreBuild` delegate