Sets the text of tooltip to be the given string.
Also see [[email protected]_text].
This is a convenience property which will take care of getting the tooltip shown if the given string is not
null: [[email protected]:has-tooltip] will automatically be set to true and there will be
taken care of [[email protected]:GtkWidget:query-tooltip] in the default signal handler.
Note that if both [[email protected]:tooltip-text] and [[email protected]:tooltip-markup] are set, the last one wins.