GApplication: Add g_application_add_main_option

This function adds a single main option entry to be handeled by
GApplication. The option entry has it arg_data field set to NULL
and will be added to the applications packed_options.

The rationale for this is that bindings will be able to add
command line options even when they can't use the un-boxed struct
GOptionEntry.

https://bugzilla.gnome.org/show_bug.cgi?id=727455