Redo header bar decorations once more
Applications need a way to fix or adapt the decoration layout, for situations like split header bars. Setting the layout from the theme with a style property did not offer a good way to do this, and the ::show-close-button property does not provide fine-grained control. To improve the situation, move the layout string to a property of GtkHeaderBar which is backed by a setting. This allows platforms to set a default button layout independent of the theme, while applications can override the default. The style GtkWindow style property is now deprecated and ignored.
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