GUACAMOLE-773: Consider Pickr available only if initialization has completed.

In previous versions of Pickr, catching exceptions during Pickr creation
was sufficient to detect whether Pickr can be used with the current
browser. This is no longer the case, and we must instead rely on the
lack of an "init" event.

guacamole/src/main/frontend/src/app/form/services/colorPickerService.js

@@ -202,7 +202,7 @@ angular.module('form').provider('colorPickerService', function colorPickerServic
          *     selectColor(), false otherwise.
          */
         service.isAvailable = function isAvailable() {
-            return !!pickr;
+            return pickrInitComplete;
         };
 
         /**