Add a new "pagesdestroy" event, dispatched *before* the BaseViewer removes an existing document

This new event essentially mirrors the existing "pagesinit" event, and will allow e.g. a custom implementation of the viewer to be notified before the current PDF document is removed from the viewer.

By using this new event, we're thus able to dispatch a "pageclose" event for JavaScript actions when closing the existing document.

web/base_viewer.js

@@ -460,6 +460,8 @@ class BaseViewer {
    */
   setDocument(pdfDocument) {
     if (this.pdfDocument) {
+      this.eventBus.dispatch("pagesdestroy", { source: this });
+
       this._cancelRendering();
       this._resetView();
 
@@ -1555,6 +1557,10 @@ class BaseViewer {
     eventBus._on("pagesinit", () => {
       dispatchPageOpen(this._currentPageNumber);
     });
+
+    eventBus._on("pagesdestroy", () => {
+      dispatchPageClose(this._currentPageNumber);
+    });
   }
 }