tool item group: Don't leak a button
tool item group: Don't leak a button

This causes a hickup in the new css node machinery, causing
the default-value test to fail.

alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk