Use GTK color picker instead of R, G, B and D buttons

Use GTK color picker instead of R, G, B and D buttons

By an awful hack in the generic dialog constructor ever Button Option
with the name R, G or B is now completely ignored, while Buttons with
name D will be replaced by GtkColorButtons. Instead of catching the
"clicked" signal these will respond to the "color-set" signal, invoking
a handler private to the GTK front end, which will set the color text
and colorof the reset button as the platform-independent handler of the
RGBD buttons would normally have done.
Unfortunately this had to use deprecated GTK2 stuff, as the recommended
GTK3 equivalents are not understood on Ubuntu 10.04.