Checkboxes and Radiobuttons need to pass block on to selection

Signed-off-by: Chris Riddoch <riddochc@gmail.com>
Provide documentation example for checkboxes yielding to a block

Also, properly test the code in the documentation
...And do a simple checkboxes_yield in the tests.

Signed-off-by: Chris Riddoch <riddochc@gmail.com>
Test :sorry and :error types of message boxes

Signed-off-by: Chris Riddoch <riddochc@gmail.com>