gi-gtk-3.0.12: ChangeLog.md

### 3.0.12

Fix a mistake in the introspection data in [widgetGetParentwindow](https://hackage.haskell.org/package/gi-gtk/docs/GI-Gtk-Objects-Widget.html#v:widgetGetParentWindow), where the return value was not properly annotated as nullable. Fixes [#90](https://github.com/haskell-gi/haskell-gi/issues/90).