Application, ApplicationCommandLine: Wrap API that uses VariantDict.

* gio/src/application.hg: Add the handle-local-options signal.
* gio/src/applicationcommandline: Add get_options_dict().
* tools/m4/convert_glib.m4: Add necessary conversions.

I really need to try using this API so we can be sure that it
is correct before calling it stable, because this part of the awful
GApplication documentation is particularly awful.