Remove button and other nonsensical child elements from Cloze

Button is no InputElement which causes trouble in some places. Since it 
does not make much sense in the Cloze context it is just removed.

projects/common/models/compound-elements/cloze-element.ts

-import { CompoundElement, InputElementValue, UIElement } from '../uI-element';
+import {
+  CompoundElement, InputElement, InputElementValue, UIElement
+} from '../uI-element';
 import { ClozePart, LikertColumn, LikertRow } from '../../interfaces/UIElementInterfaces';
 import { TextElement } from '../text-element';
 import { ButtonElement } from '../button-element';
@@ -74,9 +76,6 @@ export class ClozeElement extends CompoundElement {
 
   private static getNextSpecialElement(p: string): [number, string] {
     const x = [];
-    if (p.indexOf('\\b') > 0) {
-      x.push(p.indexOf('\\b'));
-    }
     if (p.indexOf('\\d') > 0) {
       x.push(p.indexOf('\\d'));
     }
@@ -90,7 +89,6 @@ export class ClozeElement extends CompoundElement {
     const y = Math.min(...x);
     let nextElementType = '';
     switch (p[y + 1]) {
-      case 'b': nextElementType = 'button'; break;
       case 'd': nextElementType = 'dropdown'; break;
       case 'i': nextElementType = 'text-field'; break;
       case 'z': nextElementType = 'drop-list'; break;
@@ -103,12 +101,6 @@ export class ClozeElement extends CompoundElement {
     const elementModel: UIElement = { type: elementType } as UIElement;
     let newElement: InputElement;
     switch (elementModel.type) {
-      case 'text':
-        newElement = new TextElement(elementModel);
-        break;
-      case 'button':
-        newElement = new ButtonElement(elementModel);
-        break;
       case 'text-field':
         newElement = new TextFieldElement(elementModel);
         (newElement as TextFieldElement).label = '';
@@ -122,24 +114,6 @@ export class ClozeElement extends CompoundElement {
       case 'dropdown':
         newElement = new DropdownElement(elementModel);
         break;
-      case 'radio':
-        newElement = new RadioButtonGroupElement(elementModel);
-        break;
-      case 'image':
-        newElement = new ImageElement(elementModel);
-        break;
-      case 'audio':
-        newElement = new AudioElement(elementModel);
-        break;
-      case 'video':
-        newElement = new VideoElement(elementModel);
-        break;
-      case 'likert':
-        newElement = new LikertElement(elementModel);
-        break;
-      case 'radio-group-images':
-        newElement = new RadioGroupImagesElement(elementModel);
-        break;
       case 'drop-list':
         newElement = new DropListElement(elementModel);
         newElement.height = 30;