split out nautilus-callback code

This factored-out from !1451.

It contains:

• the uncontroversial unsatifisfied/ready callback split from !1451
• preparational commits that move callback code out into dedicated functions
• splitting out of nautilus-request from nautilus-directory-async.c, so it can be used from other files
• splitting out of nautilus-callback from nautilus-directory-async.c (via file duplication git shenanigans), to make callback code comprehensible
• separating callback related parts of NautilusDirectory into a separate struct with its own memory management.