gapplication: Make sure --help output is translated

Currently, applications using g_application_add_main_option_entries()
won't get translated entries in --help output. We need to call
g_option_group_set_translation_domain() with a NULL domain to ensure that the
default application gettext domain (ie the one passed to the
textdomain() call) will be used for the main entries passed by the
application.

If we want to allow more flexibility on which gettext domain should be
used for these entries, new API will be needed.

https://bugzilla.gnome.org/show_bug.cgi?id=750322