Parses the given string for underscores and converts the next
character to an underlined character. The last character that
was underlined will have its lower-cased accelerator keyval returned (i.e.
"_File" would return the keyval for "f". This is
probably only used within the GTK+ library itself for menu items and such.
WARNING: gtk_label_parse_uline is deprecated and should not be used in newly-written code. Use gtkLabelSetUseUnderline instead.