CheckBoxGroup/OptionsGroup improvements

[sukhova]

Description of the bug or enhancement

• CheckBoxGroup
  If you set required = true, then there is no highlighting, but notification appears.
  If you select any value and then cancel your choice, the component considers then some choice was made, and save this empty value.

• OptionsGroup (multiple)
  If you set required = true, then there is no highlighting and no notification.

Conclusion

In my opinion if the “choice” component has the required=true attribute it means that it is necessary to select at least one value from the list.

[Flaurite]

Bug with canceling value has already fixed here: cuba-platform/cuba#2544.