Checkbox inversion

Checkbox optiontype can work in a "inverse" mode, meaning that off is
visualized as a checked item, and vice versa. This is very practical
when user are able to EXCLUDE items, where a CHECK means the item is
excluded, even though the property name (internal) is semantically
"show_item". Show item = true or false, is more easily understood than
2x negative, like "exclude = false, means include = true.