Warn user to overwrite a old page

[jedie]

e.g.:

• Open a item in two tabs
• change one item
• goto other tab: This Version is outdated, if use change this and save this tab, the newer item will be overwritten with maybe outdated information

Store version number to every object. Fetch the current number in background with JavaScript: Inform the user if the page is outdated.
Deny to overwrite other versions: Display a form error about this?