Rather than reimplementing the same checks for every possible resource data
type, just do it once and redirect to the correct factory container.

For the libgimp API, we leave per-type functions `gimp_*_get_by_name()` (where *
can be brush|gradient|font|palette|pattern so far), but internally they all use
gimp_pdb_get_resource().

Note that eventually we want these functions to return a list of resources as it
should be possible to have several resources of a given type with the same name
(since they are made by third-party who might have had the same idea of a name).