Sets whether this should be mapped along with its when its parent is mapped and this has been shown with show.
The child visibility can be set for widget before it is added to a container with set_parent, to avoid mapping children unnecessary before immediately unmapping them. However it will be reset to its default state of true when the widget is removed from a container.
Note that changing the child visibility of a widget does not queue a resize on the widget. Most of the time, the size of a widget is computed from all visible children, whether or not they are mapped. If this is not the case, the container can queue a resize itself.
This function is only useful for container implementations and never should be called by an application.
if true, this should be mapped along with its parent.