custom.widget. custom.widget does not get destroyed when the tooltip goes
away.
By default a box with a GtkImage and GtkLabel is embedded in
the tooltip, which can be configured using gtkTooltipSetMarkup
and gtkTooltipSetIcon.gtkTooltipSetCustom(object, custom.widget)objectcustom.widgetGtkWidget, or NULL to unset the old custom widget. [ allow-none ]