New 'change' check mode

I found a problem with 'blur' checks with jQueryUI datepicker widget field: field loses it's focus before widget updates it's value and validation error displayed, though value is correct.
I've added a new 'change' check - it works nice with such widgets and behaves almost the same as 'blur'.
I maintained BC, but had to introduce a new array option 'check_mode*s*' spelled almost the same as former 'check_mode' - that can be confusing...