GFile allows for the possibility that external implementations may not
support thread-default contexts yet, via
g_file_supports_thread_contexts(). GVolumeMonitor is not yet
thread-default-context aware.
Add a test program to verify that basic gio async ops work correctly
in non-default contexts.
http://bugzilla.gnome.org/show_bug.cgi?id=579984
GResolver provides asynchronous (and synchronous-but-cancellable) APIs
for resolving hostnames, reverse-resolving IP addresses back to
hostnames, and resolving SRV records. Part of #548466.
