Make InputElement's label prop optional

...-elements/cloze/cloze-child-elements/text-field-simple.ts

...ound-elements/cloze/cloze-child-elements/toggle-button.ts

projects/common/models/elements/element.ts

projects/common/models/elements/input-elements/checkbox.ts

projects/common/models/elements/input-elements/drop-list.ts

...els/elements/input-elements/radio-button-group-complex.ts

...mmon/models/elements/input-elements/radio-button-group.ts

...es-tab/input-groups/input-element-properties.component.ts