Offer a uiSchema option to hide labels

[jobchong]

This mostly is for array items, which do not get the label hide objects do.

When this isn't available, a nav tab containing array items will display the default uiSchema title as determined by react-jsonschema-form, which is sometimes undesirable based on how one customises the array field UI.

This option removes some unnecessary repetition in labels, because the nav title already serves as one.