Change all element models to proper classes

projects/common/classes/element.ts

projects/common/classes/unit.ts

...cts/common/components/compound-child-overlay.component.ts

...components/text-marking-bar/text-marking-bar.component.ts

projects/common/components/ui-elements/button.component.ts

projects/common/components/ui-elements/checkbox.component.ts

projects/common/components/ui-elements/cloze.component.ts

...mmon/components/ui-elements/drop-list-simple.component.ts

...ects/common/components/ui-elements/drop-list.component.ts

projects/common/components/ui-elements/dropdown.component.ts

projects/common/components/ui-elements/frame.component.ts

...onents/ui-elements/likert-radio-button-group.component.ts

projects/common/components/ui-elements/likert.component.ts

...on/components/ui-elements/radio-button-group.component.ts

...on/components/ui-elements/radio-group-images.component.ts

projects/common/components/ui-elements/slider.component.ts

.../common/components/ui-elements/spell-correct.component.ts

...ects/common/components/ui-elements/text-area.component.ts

...mon/components/ui-elements/text-field-simple.component.ts

...cts/common/components/ui-elements/text-field.component.ts