GUI: click-to-show for unrecognized crypto parts

The signatures were being shown in an expanded attachment view because
their MIME type typically inherits from text/plain. This patch makes
sure that all encrypted parts which we cannot decrypt and also the
unsupported message signatures are hidden behind a click-to-show
AttachmentView.

Change-Id: Ie6441663edbdb5deff937c10432bc3fa1bf89878