Add some TODOs about VariantDict.

We should remove Application::add_main_option_entries() and
add_option_group() if we cannot get the handle_local_entries signal
and its VariantDict right.
extra_defs: Add some new gio classes.

* tools/extra_defs_gen/generate_defs_gio.cc:
* tools/extra_defs_gen/generate_defs_glib.cc:
  Mention some recently-added GTypes.
* gio/src/gio/gio_signals.defs: Regenerate.