Resolved - Integer type of preferences are not updated

Reason: IntegerControl assumes the model, passed to it, would always has errorModel variable set properly.

In order to resolve it, now using pgBrowser.DataModel instead of Backbone.Model, which initialize the errorModel by default.

Fixes #1868

web/pgadmin/preferences/templates/preferences/preferences.js

@@ -35,7 +35,7 @@ define(
            * This model will be used to keep tracking of the changes done for
            * an individual option.
            */
-          var PreferenceModel = Backbone.Model.extend({
+          var PreferenceModel = pgBrowser.DataModel.extend({
             idAttribute: 'id',
             defaults: {
               id: undefined,