app: factor out utility function gimp_get_fill_params()

which returns an error if there is no pattern for GIMP_FILL_PATTERN.
Use it instead of having the same code in 3 variants, and make error
handling consistent with bucket fill.
app: oops, fix last commit
app: make the gimp_drawable_fill*() API symmetric to gimp_edit_fill*()
po: add app/core/gimp-utils.c to POTFILES.in

Also sort all sections alphabetically.