[Editor] Dispatch an event when some global states are changing

- this way the context menu in Firefox can take into account what we
  have in the clipboard, if an editor is selected, ...
- when the user will click on a context menu item, an action will be
  triggered, hence this patch adds what is required to handle it;
- some tests will be added in the Firefox' patch.

web/app.js

@@ -186,6 +186,10 @@ class DefaultExternalServices {
   static get isInAutomation() {
     return shadow(this, "isInAutomation", false);
   }
+
+  static updateEditorStates(data) {
+    throw new Error("Not implemented: updateEditorStates");
+  }
 }
 
 const PDFViewerApplication = {
@@ -1954,6 +1958,12 @@ const PDFViewerApplication = {
       eventBus._on("fileinputchange", webViewerFileInputChange);
       eventBus._on("openfile", webViewerOpenFile);
     }
+    if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+      eventBus._on(
+        "annotationeditorstateschanged",
+        webViewerAnnotationEditorStatesChanged
+      );
+    }
   },
 
   bindWindowEvents() {
@@ -3076,6 +3086,10 @@ function beforeUnload(evt) {
   return false;
 }
 
+function webViewerAnnotationEditorStatesChanged(data) {
+  PDFViewerApplication.externalServices.updateEditorStates(data);
+}
+
 /* Abstract factory for the print service. */
 const PDFPrintServiceFactory = {
   instance: {