added rk.askYesNo() and marked rk.show.question() as deprecated

• the arguments of rk.askYesNo() are modelled after the askYesNo() function introduced with R 3.5
• but there are some differences: the first argument is called "message" (not "msg") like in rk.show.question(), it still supports the "caption" argu
• in contrast to rk.show.question(), it returns NA if cancelled, not NULL