Create ggplot_build2 function to return prestats.data

Decided to go for this approach after fruitful discussions with @wch from ggplot2.

@tdhock @sckott Could you please have a look?
I shouldn't be able to just call `ggplot_build2()` (as I do in the test) since I haven't exported it...  Should I?
I've run `devtools::check()` but right now it is failing for other reasons, it seems... (docs-related, not code-related).  Thank you!!

/cc @xsaintmleux for good measure ;)