Consider implementing overlay-based progress indicators

[dpikhulya]

The CommandMessage component can disable its fields when the command is being posted (until the desired completion event is emitted in response). This issue is for implementing another popular way of disabling the UI while server communication is in progress -- displaying the semitransparent (or even opaque) overlay with some animated progress indicator that covers the respective piece of UI and prevents the user from interacting with it.

It still needs to be decided how exactly this has to be implemented from the API standpoint. Preliminarily it could mean implementing the following in particular:

• Implement a reusable/configurable overlay component, which is easy to integrate in popular scenarios.
• Consider supporting some easy means of integrating it with CommandForm and/or other components that utilize server communication, such as CommandDialog and CommandWizard.