Move unit interface file to common folder.

projects/common/unit.ts

...ew/page-view/canvas/canvas-element-component.directive.ts

...ts/unit-view/page-view/canvas/canvas-section.component.ts

...nents/unit-view/page-view/canvas/page-canvas.component.ts

...app/components/unit-view/page-view/page-view.component.ts

...ts/unit-view/page-view/properties/properties.component.ts

...ditor/src/app/components/unit-view/unit-view.component.ts

projects/editor/src/app/model/UnitFactory.ts

projects/editor/src/app/unit.service.ts