[editor] Add toggle-button to TextEditor in Cloze-mode

Also improve tooltips.

projects/editor/src/app/text-editor/rich-text-editor.component.html

@@ -245,14 +245,18 @@
     </button>
   </div>
   <div *ngIf="showCloseElements" fxLayout="row" fxLayoutAlign="space-around center" >
-    <button mat-icon-button matTooltip="\i" [matTooltipShowDelay]="300"
+    <button mat-icon-button matTooltip="Eingabefeld (\i)" [matTooltipShowDelay]="300"
             (click)="insertSpecialChar('\\i')">
       <mat-icon>text_fields</mat-icon>
     </button>
-    <button mat-icon-button matTooltip="\z" [matTooltipShowDelay]="300"
+    <button mat-icon-button matTooltip="Ablegefeld (\z)" [matTooltipShowDelay]="300"
             (click)="insertSpecialChar('\\z')">
       <mat-icon>drag_indicator</mat-icon>
     </button>
+    <button mat-icon-button matTooltip="Optionsfeld (\r)" [matTooltipShowDelay]="300"
+            (click)="insertSpecialChar('\\r')">
+      <mat-icon>radio_button_checked</mat-icon>
+    </button>
   </div>
 </div>
 <tiptap-editor [editor]="editor" [ngModel]="text"