GtkCellLayout: Use the new helpers
GtkCellLayout: Use the new helpers

Instead of issuing g_warning, fill the provided GError.
This lets us test this error handling, and is the right
thing to do. Use the new GtkBuilder helpers and
g_markup_collect_attributes to do so.

jadahl/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,jigpu/gtk,chergert/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,davidgumberg/gtk,Adamovskiy/gtk,jigpu/gtk,grubersjoe/adwaita,Adamovskiy/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,Adamovskiy/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,chergert/gtk,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,chergert/gtk,Adamovskiy/gtk,jigpu/gtk,davidgumberg/gtk,jigpu/gtk