[Editor] Allow to change a preference from the editor UI manager

We want to be able to update a global pref (i.e. enableAltText) in using a toggle button in the new alt-text dialog.

web/app.js

@@ -1942,6 +1942,7 @@ const PDFViewerApplication = {
         { signal }
       );
       eventBus._on("reporttelemetry", webViewerReportTelemetry, { signal });
+      eventBus._on("setpreference", webViewerSetPreference, { signal });
     }
   },
 
@@ -3166,4 +3167,8 @@ function webViewerReportTelemetry({ details }) {
   PDFViewerApplication.externalServices.reportTelemetry(details);
 }
 
+function webViewerSetPreference({ name, value }) {
+  PDFViewerApplication.preferences.set(name, value);
+}
+
 export { PDFViewerApplication };