overrides: Make value argument to Widget.style_get_property optional

Override Gtk.Widget.style_get_property to optionally accept the "value"
argument. If "value" is not supplied, the override will locate the child
property value type and create the GValue. Additionally return the resulting
GValue converted to a native Python value.

https://bugzilla.gnome.org/show_bug.cgi?id=685076