Validation option to set component values as unique

This is much needed, thanks for the code. I'm experiencing an issue, however. Once you set this to unique, you can no longer edit any entries, since when it tries to save it finds the current entry and prevents saving. It needs an extra check to see if the data being saved is from an existing submission. If so, maybe it doesn't need to check for a unique email, or at least check that the result found is something other than the current submission.

Generalizing this so that it should apply to any component type (such as textfield or select).

I've restructured Webform's validation slightly so that it now has a dedicated section on the component edit form. This should set us up nicely for features like #260708: Number component (Numeric field) and #496550: Webform element validation system.