[editor] Fix z-index

The elements z-index is no longer ignored.
Isolation helps for having negative z-indices. Those elements no longer 
are hidden behind parent elements.
The helper grid need to have a large negative z-index to not interfere.

projects/editor/src/app/components/canvas/dynamic-section-helper-grid.component.ts

 import { CdkDragDrop } from '@angular/cdk/drag-drop';
 import {
-  Component, ElementRef, EventEmitter, Input, OnChanges, OnInit, Output, SimpleChanges
+  Component, EventEmitter, Input, OnChanges, OnInit, Output, SimpleChanges
 } from '@angular/core';
 import { UIElement, UIElementType } from 'common/models/elements/element';
 import { Section } from 'common/models/section';
@@ -29,9 +29,9 @@ import { UnitService } from '../../services/unit.service';
     <ng-content></ng-content>
   `,
   styles: [
-  '.grid-placeholder {border: 5px solid aliceblue; color: lightblue; text-align: center;}'
+    '.grid-placeholder {border: 5px solid aliceblue; color: lightblue; text-align: center; z-index: -100;}'
   ]
-  })
+})
 export class DynamicSectionHelperGridComponent implements OnInit, OnChanges {
   @Input() autoColumnSize!: boolean;
   @Input() autoRowSize!: boolean;

projects/editor/src/app/components/canvas/section-dynamic.component.ts

@@ -25,12 +25,14 @@ import { DynamicSectionHelperGridComponent } from './dynamic-section-helper-grid
          [gridRowSizes]="section.gridRowSizes"
          [section]="section"
          [sectionIndex]="sectionIndex"
+         [style.isolation]="'isolate'"
          (transferElement)="transferElement.emit($event)">
 
       <!-- Angular content projection is used in the helper grid component, where the following
            is the content.-->
       <aspect-dynamic-canvas-overlay *ngFor="let element of section.elements"
                                      #elementComponent
+                                     [style.z-index]="element.position.zIndex"
                                      [element]="$any(element)"
                                      [style.margin-left]="[element.position.marginLeft] | measure"
                                      [style.margin-right]="[element.position.marginRight] | measure"