完善图层管理功能

src/core/Loader.Base.js

  */
 
 import { Util, DomUtil, PlotUtil } from './utils'
-import { MouseEventType, SceneEventType, Event } from './event'
+import {
+  MouseEventType,
+  SceneEventType,
+  LayerEventType,
+  OverlayEventType,
+  Event
+} from './event'
 import { LayerType, Layer } from './layer'
 import { OverlayType, Overlay } from './overlay'
 import ImageryType from './imagery/ImageryType'
   OverlayType,
   MouseEventType,
   SceneEventType,
+  LayerEventType,
+  OverlayEventType,
   WidgetType,
   MouseMode,
   Layer,

src/core/layer/LayerGroup.js

    */
   _onRemove() {
     Object.keys(this._cache).forEach(key => {
-      this._viewer && this._viewer.remove(this._cache[key])
+      this._viewer && this._viewer.removeLayer(this._cache[key])
     })
     this._cache = {}
     this._state = State.REMOVED

src/core/overlay/Overlay.js

    * @private
    */
   _onAdd(layer) {
-    if (!layer || !this._delegate) {
+    if (!layer) {
       return
     }
     this._layer = layer
     this._mountedHook && this._mountedHook()
     // for Entity
-    if (this._layer?.delegate?.entities) {
+    if (this._layer?.delegate?.entities && this._delegate) {
       this._layer.delegate.entities.add(this._delegate)
-    } else if (this._layer?.delegate?.add) {
+    } else if (this._layer?.delegate?.add && this._delegate) {
       // for Primitive
       this._layer.delegate.add(this._delegate)
     }