libgimpbase: don't allow parasites with zero-length names

Check in gimp_parasite_new() and fail GValue validation for parasites
with empty name. So far we only disallowed NULL names, this change
forbids the empty string "" too.
app: add a small infrastructure to validate known parasites

when they are added to items, images or globally, from the PDF or an
XCF file. None of the validation functions does anything currently,
they simply return TRUE.