trivial: Fix the return code of asb_string_replace()
trivial: Check for required values when running asb_context_setup()
trivial: Do not try to write the icons tarball if there are no icons
trivial: Fix a small memory leak in the gettext plugin
trivial: Use _cleanup_ in more places
trivial: Remove the dep-loop between AsbPluginLoader and AsbContext

Only take a weak ref when holding the context.
trivial: Do not write a log if LogDir is not set
trivial: Do not blacklist apps that were released the same day

RPM returns us a UNIX epoch set at 12:00am, so if we build the metadata before
this on a very new package, we detect it over 100 years old due to the overflow.
Do not try to download screenshots when run with --nonet
Add a self test framework for libappstream-builder