Change button label and icon to generic button

Since buttons can now act as switches for text-element highlighting, it 
is no longer exclusively used for navigation purposes.

projects/common/models/elements/button/button.ts

@@ -6,7 +6,7 @@ import { ButtonComponent } from 'common/components/button/button.component';
 import { ElementComponent } from 'common/directives/element-component.directive';
 
 export class ButtonElement extends UIElement {
-  label: string = 'Navigationsknopf';
+  label: string = 'Knopf';
   imageSrc: string | null = null;
   asLink: boolean = false;
   action: null | ButtonAction = null;

projects/editor/src/app/components/new-ui-element-panel/ui-element-toolbox.component.html

@@ -124,7 +124,7 @@
     </mat-expansion-panel-header>
     <button mat-stroked-button (click)="addUIElement('button')"
             draggable="true" (dragstart)="$event.dataTransfer?.setData('elementType','button')">
-      <mat-icon>navigation</mat-icon>
+      <mat-icon>smart_button</mat-icon>
       {{'toolbox.button' | translate }}
     </button>
     <button mat-stroked-button (click)="addUIElement('frame')"