gsubprocess: Add UTF-8 variants of communicate()

gsubprocess: Add UTF-8 variants of communicate()

Over many years of writing code interacting with subprocesses, a pattern
that comes up a lot is to run a child and get its output as UTF-8, to
put inside a JSON document or render in a GtkTextBuffer, etc.

It's very important to validate at the boundaries, and not say deep
inside Pango.

We could do this a bit more efficiently if done in a streaming fashion,
but realistically this should be OK for now.