[FEATURE REQUEST] gio::Application signal handlers should return glib::ExitCode instead of i32

[pjungkamp]

The return value for gio::Application signal handler like the closures attached using connect_command_line or connect_handle_local_options describe an glib::ExitCode but actually return an i32. I find myself writing glib::ExitCode::SUCCESS.value() in these signal handlers, which doesn't seem to be ideal...