Fix bug during title making in full upload run

To make the title, we pass the metadata dictionary in order to
match it in the title formats. Following a restructuration, the
dictionary is contained in photo.metadata, not photo. This was
fixed in single upload mode but not in full upload mode.
Add special handling for categories in post processing

During post processing, we both transform the metadata field values
and havest possible categories. They need to be passed along at the
record level, without redundancy.
Add post-processor to post-processing methods

We need to be able to pass a post processor to the various
post processing methods (quite obvisouly)
Revamp post-pocessing method wrapping signature

We assumed in the past the post processing methods
had to be related to a MetadataMapper, which may not be the case
and is actually quite cumbersome − for example, a regex method
to search and replace patterns need not to be.

Here we implement a workaround for that, where we can either pass
the name of the MetadataMapper method OR an actual method (with
arguments by the way).
Ultimately, the reliance on the MetadataMapper should go.