ComputedUpdater doesn't reevaluate the enabled property in an IAction object

[RomanTsukanov]

https://plnkr.co/edit/GC9hytDLvNTZsdEZ

In this example, a custom Clear Survey button is added to the toolbar. ComputedUpdater is used to activate the button when the survey is not empty. However, this doesn't work.

Steps to reproduce:

1. Click the "Clear Survey" button. The button becomes disabled, which is fine.
2. Add a new question to the survey.
   Expected result: The "Clear Survey" button becomes enabled again, since the survey contains an element.
   Current result: "Clear Survey" is disabled.

[tsv2013]

The creator.survey.isEmpty actually checks pages lenght. ComputedUpdater right now doesn't react on array changes, it observes whole values assignment only. Unfortunately right now for such a cases we need to use another means, like survey.registerFunctionOnPropertyValueChanged("pages", () => {});