GUI: draw a border around attachments

This comes in handy when the attachments are deeply nested; it makes it possible
to orient oneself in them again.
GUI: remove decorators around message/822 widgets

Now that they are wrapped in the generic AttachmentView, it makes little sense
to show more visual headers.
GUI: disable clickthrough for compound types

Previously, inlined compound types (like message/rfc822) required a clickthrough
before they were shown.