If the frame's label widget is a GtkLabel, returns the
text in the label widget. (The frame will have a GtkLabel
for the label widget if a non-NULL argument was passed
to gtk_frame_new().)
the text in the label, or NULL if there was no label widget or the lable widget was not a GtkLabel. This string is owned by GTK+ and must not be modified or freed.
If the frame's label widget is a GtkLabel, returns the text in the label widget. (The frame will have a GtkLabel for the label widget if a non-NULL argument was passed to gtk_frame_new().)