Returns the URI for the currently active link in the label.
The active link is the one under the mouse pointer or, in a selectable label, the link in which the text cursor is currently positioned.
This function is intended for use in a [signal@Gtk.Label:
GtkLabel:activate-link] handler or for use in a [signal@Gtk.Widget:
the currently active URI or null if there is none. The string is owned by GTK and must not be freed or modified.