A subsequent commit will change the location where edits are stored for
local items. In order to provide backwords compatibility with existing
edits stored at the old location, and to transparently migrate them to
the new location, it is necessary that a PhotosBaseItem sub-class be
able to return multiple pipeline locations.

All parameters that accepted a single pipeline path or URI have been
modified to accept a NULL-terminated array of paths or URIs.

The automatic mapping from an "as" GVariant to the natural GStrv C type
has been disabled for the GenerateThumbnail()[pipeline_uris] parameter
because the gdbus-codegen:ed wrapper cannot convert a NULL GStrv to its
equivalent GVariant. It uses g_variant_new_strv with the length always
set to -1, which isn't valid when the GStrv is NULL.

#87