gobject: new g_set_weak_pointer() & g_clear_weak_pointer() helpers

Weak-pointers are currently lacking g_set_object() & g_clear_object()
helpers equivalent. New functions (and macros, both are provided) are
convenient in many case, especially for the property's notify-on-set
pattern:

  if (g_set_weak_pointer (...))
    g_object_notify (...)

Inspired by Christian Hergert's original implementation for
gnome-builder.

https://bugzilla.gnome.org/show_bug.cgi?id=749527

docs/reference/gobject/gobject-sections.txt

@@ -279,6 +279,8 @@ g_object_weak_ref
 g_object_weak_unref
 g_object_add_weak_pointer
 g_object_remove_weak_pointer
+g_set_weak_pointer
+g_clear_weak_pointer
 GToggleNotify
 g_object_add_toggle_ref
 g_object_remove_toggle_ref