Add a filepath argument to load/save pre/post.
Also add save_post_failed and load_post_failed handlers so it's always
possible to for the pre handlers to run a matching post action.
This makes it possible to know the filepath of the blend file mean
loaded/saved as well as supporting running an action when load/save
operations fail.
When loading and saving the startup-file, the path argument is set to
an empty string.
Details:
New RNA types were added to support storing primitive values in
PointerRNA. Primitive{String/Int/Float/Boolean}RNA. These will likely
only be used in some limited cases, in the case of BKE_callback_exec it
allows strings to be included as part of the PointerRNA **pointers
argument.
Ref !104769.