minor improvements to form labels

the `LabelOptionsAwareInterface` was introduced in #4677 to preserve BC with 5.3.3, but since the whole functionality will make into 2.3 release and we're upping the minimum PHP version, some cleaning is due.

also minor cleanup subsequent to #5533 and reinstated #5539 new feature

it is important to merge this along with #4677 (which hasn't made yet into any release) before 2.3, otherwise this would become a bc break for the very PR it's trying to clean up. ;)