New optional widget: GTK.Databox

Rev: src/post_modules/GTK/Makefile.in:1.18
Rev: src/post_modules/GTK/build_pgtk.pike:1.26
Rev: src/post_modules/GTK/configure.in:1.13
Rev: src/post_modules/GTK/make_example_image.pike:1.5
Rev: src/post_modules/GTK/options.in:1.7
Rev: src/post_modules/GTK/source/gtkdatabox.pre:1.1