Usability: Notification when page URL is already used

[ischommer]

If you rename a URL to a string thats already used somewhere else, the system doesn't alert you. In fact, it reports you wrong information.

• Create a page with URL "home", Save, Publish
• Create a second page with URL "home", Save, Publish ("Changed URL to 'home')
• Refresh second page => URL is set to "home-2"

Suggest that we implement URL as an UniqueTextField? that automatically checks if the name is already used via ajax. In case the user is faster with saving than the ajax-response, a server-side validation-error is returned.