glib: add a proper weak-ref-like relationship between GSource and GMainContext

Like the GWeakRef's in GObject, there is a global lock that is consulted
whenever g_main_context_unref() or g_source_destroy() or
g_source_unref() is called to retrieve a reference to the associated
GMainContext.

There are a number of actual races that are fixed by this change.
1. Racing GSource destruction with g_main_context_unref() is solved
   by holding the global source_weak_locations lock while setting
   source->context = NULL and while g_source_destroy() attempts to
   retrieve source->context;
2. Same race as 1. but inside g_source_unref()
3. Theoretical race of double freeing the contents of
   context->pending_dispatches if both g_source_destroy() and
   g_main_context_unref() both enter g_main_context_unref().

A couple of implementation notes:
1. Unlocking source_weak_locations too early in g_main_context_unref()
   (before g_source_destroy_internal() is called) may have a race of the
   G_HOOK_FLAG_ACTIVE state of the source and cause a leak of the source.
   This is why source_weak_locations is also held over the calls to
   g_source_destroy_internal() in g_main_context_unref().  So that
   either g_main_context_unref() or g_source_destroy()
2. g_main_context_unref() now needs to be more of a dispose()
   implementation as it can be called multiple times with losing the
   last ref.

Fixes: GNOME/glib#803