girparser: Move special handling for certain parameters to process_callable()
girparser: Move special handling for certain parameters to process_callable()

- Detect delegate throwing error when GLib.Error out-parameter exists
- Mark method as async based on finding an AsyncReadyCallback parameter
- Apply null-literal default-value for trailing GLib.Cancellable? parameters

GNOME/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,frida/vala