Remove obsolete form group in mobileshowcase
[BUG #8730] Add validator to date picker and change the order of date picker config to ensure that the validator doesn't validate not before required is set
[BUG #8730] Remove obsolete date picker callback to check value on selected event. This isn't required anymore because it will validate within the given validator
[BUG #8730] Leave date string in date picker as it is and don't transform it to locale date string. This is necessary to validate the date correctly