Sets whether the hexpand flag (see Widget.getHexpand) will be used.
The hexpand-set property will be set automatically when you call Widget.setHexpand to set hexpand, so the most likely reason to use this function would be to unset an explicit expand flag.
If hexpand 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.