We rely on the parent pointer to determine if the widget is being
hovered over. Set the parent pointer so that the background is set
properly for a widget in a conditional container.
A common pattern when using libtextscreen is to want to show or hide
particular UI components depending on the values of particular
variables. It is also common to want to only add things to the UI if
a particular boolean condition is true. Add a new conditional widget
to cover the first case, and a convenience TXT_If() function for the
latter.