Should we relax requirement from gtk 2.6 to gtk 2.4.

[David Koppelman]

Recently Emacs changed the gtk version from 2.4? to 2.6, perhaps to
accommodate a use of gtk_image_new_from_icon_name and
gtk_image_set_from_icon_name. If that's all, then perhaps it would be
better to use 2.4-compatible functions to do the same thing.