Sets the text of tooltip to be the given string, which is marked up with Pango markup.
Also see gtk.tooltip.Tooltip.setMarkup.
This is a convenience property which will take care of getting the tooltip shown if the given string is not null: property@Gtk.Widget:has-tooltip will automatically be set to true and there will be taken care of signal@Gtk.Widget::query-tooltip in the default signal handler.
Note that if both property@Gtk.Widget:tooltip-text and property@Gtk.Widget:tooltip-markup are set, the last one wins.
Set tooltipMarkup property.