Gets whether set_hexpand has been used to explicitly set the expand flag on this widget.
If [property@Gtk.Widget:hexpand] property is set, then it overrides any computed expand value based on child widgets. If `hexpand` is not set, then the expand value depends on whether any children of the widget would like to expand.
There are few reasons to use this function, but it’s here for completeness and consistency.
whether hexpand has been explicitly set