Add option to allow deselection of drowdown value

projects/common/element-components/dropdown.component.ts

@@ -18,6 +18,7 @@ import { FormElementComponent } from '../form-element-component.directive';
               {{$any(elementModel).label}}
           </mat-label>
           <mat-select [formControl]="elementFormControl">
+              <mat-option *ngIf="elementModel.allowUnset" value=""></mat-option>
               <mat-option *ngFor="let option of elementModel.options" [value]="option">
                   {{option}}
               </mat-option>

projects/common/unit.ts

@@ -100,6 +100,7 @@ export interface DropdownElement extends InputUIElement, TextUIElement, SurfaceU
   label: string;
   options: string[];
   value: string | undefined;
+  allowUnset: boolean;
 }
 
 export interface RadioButtonGroupElement extends InputUIElement, TextUIElement, SurfaceUIElement {

projects/editor/src/app/UnitFactory.ts

@@ -143,6 +143,7 @@ export function createCheckboxElement(): CheckboxElement {
 export function createDropdownElement(): DropdownElement {
   return <DropdownElement><unknown>{
     options: [],
+    allowUnset: false,
     ...createUnitUIElement('dropdown'),
     ...createInputUIElement('Label Dropdown', undefined),
     ...createTextUIElement(),

projects/editor/src/app/components/unit-view/page-view/properties/element-properties.component.ts

@@ -65,6 +65,12 @@ import { SelectionService } from '../../../../selection.service';
               </mat-select>
             </mat-form-field>
 
+            <mat-checkbox *ngIf="combinedProperties.hasOwnProperty('allowUnset')"
+                          [checked]="$any(combinedProperties.allowUnset)"
+                          (change)="updateModel('allowUnset', $event.checked)">
+              Deselektion erlauben
+            </mat-checkbox>
+
             <mat-divider></mat-divider>
 
             <mat-checkbox *ngIf="combinedProperties.hasOwnProperty('required')"