Instead, add a function gtk_image_set_icon_size() for the cases where
overriding the icon size is necessary.
Treat icon sizes the same way as pixel sizes, too. So gtk_image_clear()
no longer unsets the icon size.
It's not a hugely complicated file, but it's easier to deal with some of
the details of tooltip windows styling if we have a UI file to edit,
instead of source code.
