CheckboxList & more Dialogs documentation

Squashed the following commits:

* Implement CheckboxList out of Checkbox
* Add styling reference sheet
* Document radiolist_dialog & checkboxlist_dialog
* Add standalone styled-checkboxlist example
* Add example on how to use the style sheet
* Fix typo: frame-label instead of frame.label, resulting in the style not beeing applied
* Prompt toolkit 3.0: update pages/dialogs.rst
* Apply suggestions