h5: moved the cursor style over to main css

* html5 code incorrectly adding a style sheet per instance
(and never removing them) for what should be a global style.

Author: adumesny

src/gridstack.scss

@@ -65,7 +65,6 @@ $animation_speed: .3s !default;
     &.ui-resizable-resizing {
       z-index: 100;
 
-      > .grid-stack-item-content,
       > .grid-stack-item-content {
         box-shadow: 1px 4px 6px rgba(0, 0, 0, 0.2);
         opacity: 0.8;
@@ -125,4 +124,9 @@ $animation_speed: .3s !default;
   &.grid-stack-animate .grid-stack-item.grid-stack-placeholder{
     @include vendor(transition, left .0s, top .0s, height .0s, width .0s);
   }
+
+  // without this, the html5 drag will flicker between no-drop and drop when dragging over second grid
+  &.ui-droppable.ui-droppable-over > *:not(.ui-droppable) {
+    pointer-events: none;
+  }
 }

src/gridstack.ts

@@ -563,6 +563,7 @@ export class GridStack {
       this.el.parentNode.removeChild(this.el);
     }
     this._removeStylesheet();
+    delete this.placeholder;
     delete this.engine;
     return this;
   }

src/h5/dd-droppable.ts

@@ -23,7 +23,6 @@ export class DDDroppable extends DDBaseImplement implements HTMLElementExtendOpt
   public el: HTMLElement;
   public option: DDDroppableOpt;
   private acceptable: boolean = null;
-  private style;
 
   constructor(el: HTMLElement, opts: DDDroppableOpt = {}) {
     super();
@@ -82,7 +81,6 @@ export class DDDroppable extends DDBaseImplement implements HTMLElementExtendOpt
     this.el.classList.add('ui-droppable');
     this.el.addEventListener('dragenter', this.dragEnter);
     this.setupAccept();
-    this.createStyleSheet();
     return this;
   }
 
@@ -162,20 +160,6 @@ export class DDDroppable extends DDBaseImplement implements HTMLElementExtendOpt
     return this;
   }
 
-  // TODO: share this with other instances and when do remove ???
-  private createStyleSheet(): DDDroppable {
-    const content = `.ui-droppable.ui-droppable-over > *:not(.ui-droppable) {pointer-events: none;}`;
-    this.style = document.createElement('style');
-    this.style.innerText = content;
-    this.el.appendChild(this.style);
-    return this;
-  }
-
-  // TODO: not call
-  private removeStyleSheet() {
-    this.el.removeChild(this.style);
-  }
-
   private ui(drag: DDDraggable) {
     return {
       draggable: drag.el,