the #GtkWidget to use as the contents of the page
the #GtkWidget to be used as the label for the page, or %NULL to use the default label, “page N”
the index (starting at 0) at which to insert the page, or -1 to append the page after all other pages
Return: the index (starting from 0) of the inserted page in the notebook, or -1 if function fails
Insert a page into @notebook at the given position.