behavior change: date picker, stringResult

up to 2.1.8 date picker gave as stringResult date in format Y-m-d independent of format: property.

now date picker gives stringResult in same format like defined by property format.

would be nice for compatibility reasons to have 2 properties, control format and string result format.

Behavior will be normalized in the next update
setValue and getValue will use parseFormat from webix.i18n
config.format will be used only to define format of visible text