Add g_main_context_ref_thread_default()

Add g_main_context_ref_thread_default()

Add g_main_context_ref_thread_default(), which always returns a
reffed GMainContext, rather than sometimes returning a (non-reffed)
GMainContext, and sometimes returning NULL. This simplifies the
bookkeeping in any code that needs to keep a reference to the
thread-default context for a while.

https://bugzilla.gnome.org/show_bug.cgi?id=660994