Sets the `GtkPopover` that will be popped up when the this is clicked.
If popover is null, the button is disabled.
If [[email protected]:menu-model] is set, the menu model is dissociated from the this, and the property is set to null.
| this |
a `GtkMenuButton` |
| popover |
a `GtkPopover`, or null to unset and disable the button |