[GENERIC viewer] Warn about AppOptions being overridden by Preferences during loading

Currently any AppOptions set using e.g. the "webviewerloaded" event listener can/will by default be overridden when the Preferences are read.
To avoid that happening the "disablePreferences"-option can be used, however unless it's been explicitly set all non-default AppOptions will be silently ignored. This patch thus attempts to improve the current situation somewhat, for third-party implementations, by logging a warning in the console when this happens.

web/app_options.js

@@ -379,6 +379,13 @@ class AppOptions {
   static remove(name) {
     delete userOptions[name];
   }
+
+  /**
+   * @ignore
+   */
+  static _hasUserOptions() {
+    return Object.keys(userOptions).length > 0;
+  }
 }
 
 export { AppOptions, compatibilityParams, OptionKind };