Remove hidden option from frame border styles

This complicates styling and is somewhat useless as the border can also 
be removed by setting the border-width to 0.

projects/common/ui-elements/frame/frame-element.ts

@@ -10,7 +10,7 @@ import { initPositionedElement, initSurfaceElement } from '../../util/unit-inter
 export class FrameElement extends UIElement implements PositionedElement, SurfaceElement {
   borderWidth: number = 1;
   borderColor: string = 'black';
-  borderStyle: 'hidden' | 'solid' | 'dotted' | 'dashed' | 'double' | 'groove' | 'ridge' | 'inset' | 'outset' = 'solid';
+  borderStyle: 'solid' | 'dotted' | 'dashed' | 'double' | 'groove' | 'ridge' | 'inset' | 'outset' = 'solid';
   borderRadius: number = 0;
 
   positionProps: PositionProperties;

projects/editor/src/app/components/unit-view/page-view/properties-panel/element-model-properties.component.html

@@ -549,7 +549,7 @@
     <mat-label>{{'propertiesPanel.borderStyle' | translate }}</mat-label>
     <mat-select [value]="combinedProperties.borderStyle"
                 (selectionChange)="updateModel.emit({ property: 'borderStyle', value: $event.value })">
-      <mat-option *ngFor="let option of ['hidden', 'solid', 'dotted', 'dashed',
+      <mat-option *ngFor="let option of ['solid', 'dotted', 'dashed',
                                          'double', 'groove', 'ridge', 'inset', 'outset']"
                   [value]="option">
         {{option}}