Webform confirmation message cannot be styled

Problem/Motivation

In its current form the confirmation message that is displayed after submitting a webform cannot be styled.

Because the returned text is not passed through a theming function and because the AJAX handler wraps it in a div without any classes it's impossible to style the message.

Proposed resolution

Pass the returned confirmation message through theme_webform_submission, similar to how it's done in the webform module.

User interface changes

The webform confirmation in colorbox will now use the webform_confirmation.tpl.php template.